Installation of inductance coils on simple printed circuit boards. Radio elements manufactured by printing method. Single-layer printed circuit board

In a small-sized VHF, contour coils and RF chokes occupy a relatively much space on the board. Often exactly they determine the overall height of the circuit board. In some cases, it may be appropriate to apply flat coils - printed and wires. The basis for printed RF coils is most often served by special high-frequency ceramics. The technology of production of such coils is unsuitable for amateur conditions. However, as practice shows, to the frequencies of 80-100 MHz, quite satisfactory results can be obtained using coils made from foil glass platform by etching. Application for printed coils of foil fluoroplastic allows you to move the frequency limit to 200-300 MHz. Flat wire coils have satisfactory mechanical strength, relatively small with their own capacity, ease of manufacture and can be used at frequencies to 10 MHz. An essential increase in the inductance and the quality limit of flat printed and wire coils can be obtained if the coil from one or both sides to the coil is embedded by ferrite plates. By changing the distance between the car and the plate (a set of non-magnetic gaskets or otherwise), you can change the inductance of the coil. It is possible to regulate the inductance in some limits using a non-magnetic talla (copper or aluminum) flange, moving near the coil parallel to it. Wire coils are conveniently placed directly on the board or on a separate plate, attached to the board. Print coils can be arbitrary shape. "Ground" on the board should be the outlook of the outside turn - in this case it plays the role of the screen. You can additionally shield the printed coil by another outdoor unlocked twist, connecting the machine with a shared device wire. Examples of coils are shown in the photo.

Calculate coils with an accuracy sufficient for the radio amateur using nomograms. The procedure for calculating printed and wire coils is similar, the difference is that the width of the printed path of the printed coil corresponds to the diameter of copper wire coil wires, and the width of the gap between the tracks is the double thickness of the wire isolation.

The design dimensions of coils are shown in Fig. 1, a and b. Numbers for calculation are depicted in Fig. 2 and 3. As an example, the calculation of the round printed coil (without a core) is considered by the inductance of 0.64 μg. The largest outer diameter D of the coil is chosen equal to 20 mm, the smallest internal d \u003d 8 mm. It is necessary to find the number of turns W, the width of the printed path S and the distance Sr between the centers C1 and C2 semi-rapids of the coil. The nomogram for calculating round coils is presented in Fig. 2. Calculate: d + d \u003d 20 + 8 \u003d 28 mm \u003d \u003d 2.8 cm: d / d \u003d 20: 8 \u003d 2.5. On the scales "D + D" and "D / D" we find the appropriate points and connect them straight (in fig. 2 - dash line). Through the intersection point of this straight line with a non-social auxiliary line and a point on the "L" scale, corresponding to the predetermined inductance L \u003d 0.64 μH, we carry out directly before intersection with the scale "W", according to which we count the desired number of turns - 6.5. Values \u200b\u200bD + D, D / D or L on the nomogram scales can be increased (decrease) at 10 or 100 times, while W values \u200b\u200bwill be changed to the root of 10 and the root of 100 times. The width s, mm, the printed conductor is calculated by the formula: s\u003e \u003d sr \u003d (d - d) / 4w; The diameter of the wire isolation of the wire coil - DIP \u003d (D - D) / 2W. The result obtained is rounded to the nearest majority of the row of 0.5; 0.75; 1.0; 1.25; 1.5 mm, etc. Sr \u003d (20-8) / 4x6.5 \u003d 0.46; S \u003d 0.5 mm. With small values \u200b\u200bof SR, it is necessary to receive SR \u003d S for the translucent coils of DIP, rounded to the nearest standard diameter of the isolation wire. The drawing of the coils are applied to the foil glass textolite with a circular, which is installed by a raysfededer filled with a chemically paint resistant. The upper floor of the circle (see fig. 1a) is carried out from the center C1, and the bottom - from C2. The distance Sr should withstand with possible greater accuracy. After drying, the KRAKI coil is etched, as usual, in the solution of chlorine iron. Printing coils of the square shape are calculated by the nomogram shown in Fig. 3. More accurate results of the calculation of flat coils can be obtained analytically, using formulas for which nomograms are built. These formulas are shown in Fig. 2 and 3. The dimension of the values \u200b\u200bin the formulas corresponds to a specified on the nomogram. The values \u200b\u200bof the functions "FI" (D / D and F (A / A) are summarized in table. 1 and 2. Wire flat coils are wound on the collapsible frame between two cheeks reinforced on the rod. The diameter of the frame core must be equal to the inner diameter of the coil, and The distance between the cheeks is the diameter of the wire isolation. In the process of winding, the wire is wetted by glue BF ~ 2. The cheeks should be made of a material having poor adhesion to glue (fluoroplastic, viniflex). The frame is dismantled after the end of the drying of the glue. Moorous coils are glued or directly to the board, or to the ferrite plate reinforced on the board. The coils depicted in the header of the article have the following measured parameters: round printed (D \u003d 40 mm) - inductance of 1.4 μg, Quality 95; Square (A \u003d 30 mm) - 0.9 μH and 180, wire top (d \u003d 15 mm, PEV-1 wire 0.18) - 7.5 μH and 48; Average (d \u003d 11.9 mm, wire PEV-2 0.1) - 9.5 μH and 48 and lower (d \u003d 9mm, PAL wire 0.05) - 37 μH and 43

Bilateral printed circuit boardsDespite all its advantages, are not the best, especially for non-signal or high-speed schemes. In general, the thickness of the printed circuit board, i.e. The distance between the metallization layers is 1.5 mm equals, which is too much for the complete implementation of some advantages of the two-layer printed circuit board, which are above. Distributed capacity, for example, too small due to such a large interval.

Multilayer printed circuit boards

For responsible schemechnical developments require multilayer printed circuit boards (MPP). Some reasons for their use are obvious:

the same convenient, as well as for a common wire tire, power tire layout; If the power tires use polygons on a separate layer, it is quite simple with the help of transition holes to carry out a power supply to each element of the circuit
signal layers are exempt from power tires, which makes it easier for signaling conductors
a distributed container appears between land and nutrition polygons, which reduces high-frequency noise

In addition to these reasons for the application of multilayer printed circuit boards, there are other, less obvious:

better suppression of electromagnetic ( EMI) and radio frequency ( RFI) interference due to reflection effect ( image Plane Effect.), known in the time of Marconi. When the conductor is placed close to a flat conductive surface, most of the return high-frequency currents will flow through the plane directly under the conductor. The direction of these currents will be opposite to the direction of currents in the conductor. Thus, the reflection of the conductor in the plane creates a signal transmission line. Since currents in the conductor and in the plane are equal in size and are opposite to the direction, a slight decrease in the emitted interference is created. The reflection effect effectively works only with inseparable solid polygons (they can be both land polygons and power polygons). Any integrity impairment will lead to a decrease in interference suppression.
reducing the total value with small-scale production. Despite the fact that the manufacture of multilayer printed circuit boards is more expensive, their possible radiation is less than that of single and two-layer boards. Consequently, in some cases, the use of only multilayer boards will allow the requirements for radiation set in the development, and do not conduct additional tests and testing. The use of the MPP can reduce the level of radiated interference by 20 dB compared to two-layers.

The order of the layers

Inexperienced developers often have some confusion about the optimal order of the printed circuit board. Take for example a 4-layer ward containing two signal layers and two polygon layers - layer of the Earth and a layer of power. What is the order of layers the best? Signal layers between polygons that will serve as screens? Or do the polygon layers internal to reduce the mutual influence of the signaling layers?

When solving this question, it is important to remember that often the location of the layers does not matter much, since all the same components are located on the outer layers, and the tires, bringing the signals to their conclusions, sometimes pass through all the layers. Therefore, any screen effects represent only a compromise. In this case, it is better to take care of creating a large distributed capacity between nutrition and land polygons, placing them in the inner layers.

Another advantage of the location of the signal layers outside is the availability of signals for testing, as well as the ability to modify links. Anyone who at least changed the connection of conductors located in the inner layers will appreciate this opportunity.

For printed circuit boards with more than four layers, there is a general rule to have high-speed signal conductors between land and power polygons, and low-frequency to remove external layers.

Ground

Good grounding is a general requirement of a saturated, multi-level system. And it should be planned from the first step of designer development.

Basic rule: earth separation.

The separation of land on the analog and digital part is one of the simplest and most effective methods for suppressing noise. One or more layers of a multilayer printed circuit board are usually discharged under a layer of earth ground polygons. If the developer is not very practiced or inattentive, the land of the analog part will be directly connected to these polygons, i.e. Analog recovery current will use the same chain as the digital return current. Auto suppliers work approximately and combine all the land together.

If the previously developed printed circuit board with a single earth truck, which combines analog and digital earth, is being recycled, then you must first physically divide the Earth on the board (after this operation, the board becomes almost impossible). After that, all connections to the analog landing polygon of the components of the analogue circuit are presented (analog land is formed) and to a digital earth polygon of the digital circuit components (digital land is formed). And only after that the source is combined with digital and analog land.

Other rules for the formation of the Earth:

Almost all clock frequency signals are sufficiently high-frequency signals, so even small containers between the tracks and polygons can create significant ties. It must be remembered that not only the main clock frequency can cause the problem, but also its higher harmonics.

Figure 4 shows a possible option for the placement of all components on the board, including the power source. Here are three separated from each other and insulated land / power polygon: one for the source, one for digital circuit and one for analog. The chains of the Earth and the supply of the analog and digital parts are combined only in the power source. High-grade noise is filtered off in the supply chains chokes. In this example, high-frequency signals of the analog and digital parts are related to each other. Such a design has a very high probability for a favorable outcome, since good placement of components and following chain separation rules are provided.

There is only one case when it is necessary to combine analog and digital signals over the area of \u200b\u200bthe analog land polygon. Analog-digital and digital-analog converters are placed in the housings with the outputs of the analog and digital land. Taking into account the previous arguments, it can be assumed that the output of digital land and the withdrawal of the analog land must be connected to the tires of digital and analog land, respectively. However, in this case it is not true.

The titles of the conclusions (analog or digital) refer only to the internal structure of the converter, to its internal connections. In the diagram, these conclusions must be connected to the analog land bus. The connection can be performed inside integrated circuitHowever, it is quite difficult to obtain a low resistance of such a compound due to topological restrictions. Therefore, when using converters, an external compound of the findings of the analog and digital land is assumed. If this is not done, then the parameters of the chip will be much worse than the specification.

It is necessary to take into account the fact that the digital elements of the converter may worsen the qualitative characteristics of the scheme, bringing digital interference in the chain of the analog land and analog power. When developing converters, this negative impact is taken into account so that the digital part consuming as little power as possible. In this case, interference from switching logical elements decreases. If the digital converter squeezes are not much loaded, the internal switching usually do not cause special problems. When developing a printed circuit board, containing a ADC or DAC, must be properly related to the junction digital nutrition converter to analog land.

Frequency characteristics passive components

For proper work Analog schemes are very important right choice Passive components. Start designer development with careful consideration of the high-frequency characteristics of passive components and pre-location and layouts on the sketch of the board.

A large number of developers completely ignore frequency limits of passive components when used in analog circuit engineering. These components have limited frequency ranges And their work outside the specified frequency domain can lead to unpredictable results. Someone may think that this discussion concerns only high-speed analog schemes. However, this is far from the way - high-frequency signals strongly affect the passive components of low-frequency circuits by radiation or direct communication on the conductors. For example, a simple low-frequency filter on an operating amplifier can easily turn into a high-frequency filter when exposed to its high frequency input.

Resistors

The high-frequency characteristics of the resistors can be represented by the equivalent circuit shown in Figure 5.

Usually apply resistors of three types: 1) wire, 2) carbon composite and 3) film. It is not necessary to have a lot of imagination in order to understand how a wire resistor can turn into inductance, since it is a coil with a wire of high-wing metal. Most of the developers of electronic devices do not have the concept of the internal structure of film resistors, which are also a coil, however, from a metal film. Therefore, film resistors also have inductance, which is less than that of wire resistors. Film resistors with resistance of no more than 2 kΩ can be freely used in high-frequency schemes. The conclusions of the resistors are parallel to each other, therefore there is a noticeable capacitive connection between them. For resistors with greater resistance, the inter-exterior container will reduce the full impedance at high frequencies.

Condencators

The high-frequency characteristics of the capacitors can be represented by the equivalent circuit shown in Figure 6.

Capacitors in analog circuits are used as elements of the junction and filter components. For an ideal condenser, reactive resistance is determined by the following formula:

Consequently, the electrolytic capacitor with a capacity of 10 μF will have 1.6 ohm resistance at a frequency of 10 kHz and 160 μC at a frequency of 100 MHz. Is it so?

When using electrolytic capacitors, you must follow proper connection. A positive output must be connected to a more positive constant potential. Incorrect connection leads to a flow through an electrolytic constant current capacitor, which may not only be due to the condenser itself, but also part of the scheme.

In rare cases, the difference in DC potentials between two points in the diagram can change its sign. This requires the use of non-polar electrolytic capacitors, the internal structure of which is equivalent to two polar capacitors connected in series.

Inductance

High-frequency inductance characteristics can be represented by the equivalent circuit shown in Figure 7.

Reactive inductance resistance is described by the following formula:

Consequently, the inductance of 10 mG will have a 628 ohm reactive resistance at a frequency of 10 kHz, and at a frequency of 100 MHz - the resistance of 6.28 mΩ. Right?

The printed fee itself has the characteristics of the passive components discussed above, however, not so obvious.

The pattern of conductors on the printed circuit board can be both a source and a noise receiver. A good wiring of conductors reduces the sensitivity of the analog scheme to the radiation of the sources.

The printed fee is susceptible to radiation, since the conductors and conclusions of the components form peculiar antennas. The theory of antennas is a rather complicated subject for study and is not considered in this article. However, some foundations are given here.

A little from the theory of antennas

On the constant toke. or low frequencies The active component prevails. With increasing frequency, the reactive component is becoming more and more significant. In the range from 1 kHz to 10 kHz, the inductive component begins to influence, and the conductor is no longer a low-voltage connector, but rather acts as a inductance coil.

The formula for calculating the inductance of the printed circuit board is as follows:

Usually, the tracks on the printed circuit board have values \u200b\u200bfrom 6 NGN to 12 NGN per centimeter of length. For example, a 10-centimeter conductor has a 57-MΩ resistance and an inductance of 8 NGN to see. At a frequency of 100 kHz, reactive resistance becomes 50 mΩ, and at higher frequencies the conductor will be rather inductance than the active resistance.

The pin antenna rule says that it begins to significantly interact with the field at its length about 1/20 from the wavelength, and the maximum interaction occurs at a pin length of 1/4 on the wavelength. Therefore, a 10-centimeter conductor from an example in the previous paragraph will begin to become a pretty good antenna at frequencies above 150 MHz. It must be remembered that despite the fact that the generator clock frequency The digital circuit may not work at a frequency above 150 MHz, the highest harmonics are always present in its signal. If components with high-length pins are present on the printed circuit board, then such conclusions can also serve as antennas.

Another major type of antennas - loop antennas. The inductance of the direct conductor increases greatly when it bends and becomes part of the arc. Increasing inductance reduces the frequency on which the antenna interaction with the field lines begins.

Experienced printed circuit board designers, quite well-versed in the theory of looped antennas, know that it is impossible to create a loop for critical signals. Some developers, however, do not think about it, and return and signal current conductors in their schemes are a loop. Creating loop antennas is easy to show on the example (Fig. 8). In addition, the creation of a slit antenna is shown here.

Consider three cases:

Option A is an example of a bad design. It does not use the analog land polygon at all. The loop outline is formed by an earthen and alarm conductor. When current passes, the magnetic field perpendicular to it occurs. These fields form the basis of the loop antenna. The loop antenna rule says that for the greatest efficiency, the length of each conductor should be equal to half the wavelength of the received radiation. However, we should not forget that even at 1/20 from the wavelength, the loop antenna still remains quite effective.

The variant B is better than option a, but there is a break in the test site, it is likely to create a specific place to wiring signal conductors. The paths of signal and return currents form a slotted antenna. Other loops are formed in cutouts around the chip.

Option B is an example of a better design. The paths of the signal and return current coincide, reducing the efficiency of the loop antenna. Note that in this embodiment there are also cuts around the chip, but they are separated from the return current path.

The theory of reflection and matching signals is close to the theory of antennas.

When the printed circuit board turns to an angle of 90 °, a signal may occur. This is mainly due to changes in the width of the current passage. At the top of the angle, the width of the track increases 1.414 times, which leads to the mismatch of the characteristics of the transmission line, especially distributed capacity and the own inductance of the route. Quite often, you must rotate on the printed circuit board by 90 °. Many modern CAD packets allow you to smooth out the corners of the trails carried out or conduct the tracks in the form of an arc. Figure 9 shows two steps to improve the angle shape. Only the last example supports the constant width of the track and minimizes reflections.

Council for prototypes of printed circuit boards: Leave the smoothing procedure to the last stage of work before creating drop-shaped conclusions and fill polygons. Otherwise, the CAD package will be smoothing longer due to more complex calculations.

For example, a circuit board may have the following parameters:

4 layers; Signal and layer of land polygon - adjacent
interlayer interval - 0.2 mm
explorer width - 0.75 mm
explorer length - 7.5 mm

The model value of the dielectric constant ER for FR-4 is 4.5.

It can be seen that the amplitude of the output signal is doubted at frequencies close to the upper limit of the frequency range of OU. This, in turn, can lead to generation, especially at the antenna operating frequencies (above 180 MHz).

This effect generates numerous problems to solve which, however, there are many ways. The most obvious of them is to reduce the length of the conductors. Another way is to reduce their width. There is no reason for the use of the conductor of such a width for lining the signal to the inverting input, because By this conductor proceeds very small current. Reducing the length of the track to 2.5 mm, and the widths up to 0.2 mm will lead to a decrease in the tank to 0.1 PF, and such a container will no longer lead to such a significant rise in the frequency response. Another solution is the removal of the polygon part under the inverting input and the conductor suitable for it.

The width of the printed circuit board is impossible to reduce infinitely. Limit width is defined as technological processand foil thickness. If two conductors go close to each other, then a capacitive and inductive connection is formed between them (Fig. 12).

Signal conductors should not be divorced in parallel to each other, eliminating cases of differential or microstrip lines. The gap between the conductors must be at least three times the width of the conductors.

The container between the tracks in analog schemes can create difficulties at large resistance resistors (several MOM). A relatively large capacitive bond between the inverting and non-converting inputs of the operating amplifier can easily lead to self-excitation scheme.

For example, with d \u003d 0.4 mm and H \u003d 1.5 mm (sufficiently common values) the inductance of the opening is 1.1 NGN.

Remember that if there are big resistance in the diagram, then special attention should be paid to clean the board. On the final operations of the manufacture of a printed circuit board should be removed the remains of flux and contamination. IN lately When installing printed circuit boards, water-soluble fluxes are often used. Being less harmful, they are easily removed by water. But at the same time, the washing of the board is insufficient water can lead to additional pollution, which worsen the dielectric characteristics. Consequently, it is very important to produce a printed circuit board with a high-impedance circuit with fresh distilled water.

Tie signals

As already noted, interference can penetrate the analog part of the circuit through the power chain. To reduce such interference, unleashing (blocking) capacitors that reduce the local power supply terminal are used.

If you need to breed a printed circuit board, on which there are analog and digital parts, it is necessary to have at least a small idea of \u200b\u200bthe electrical characteristics of logical elements.

The typical output stage of the logical element contains two transistors, sequentially interconnected, as well as between the supply chains and land (Fig. 14).

These transistors in the ideal case work strictly in antiphase, i.e. When one of them is open, then at the same time the second is closed, forming at the output or a logical unit, or a logical zero. In the steady logical state, the power consumption of the logical element is small.

The situation changes dramatically when the output cascade switches from one logical state to another. In this case, for a short period of time, both transistors can be opened at the same time, and the output cascade power current increases, because the resistance of the current path area from the power bus is reduced to the land bus through two successively connected transistors. Power consumed jumps like increases, and then decreases, which leads to local change supply voltage and the occurrence of a sharp, short-term change. Such current changes lead to radiofrequency energy radiation. Even on a relatively simple printed circuit board, there may be dozens or hundreds of considered output cascades of logical elements, so the total effect of their simultaneous work can be very large.

It is impossible to accurately predict the frequency range in which these current emissions will be, since the frequency of their occurrence depends on the set of reasons, including from the delay in the propagation of switching transistors of the logical element. The delay, in turn, also depends on the set of random causes arising in the production process. Noise from switching has a broadband distribution of harmonic components in the entire range. There are several ways to suppress digital noise, the use of which depends on the spectral distribution of noise.

Table 2 presents the maximum operating frequencies for common types of capacitors.

table 2

It is obvious from the table that tantalum electrolytic capacitors are used for frequencies below 1 MHz, ceramic capacitors should be applied at higher frequencies. It is necessary not to forget that the capacitors have their own resonance and their incorrect choice can not only help, but also aggravate the problem. Figure 15 shows typical own resonances of two overall application capacitors - 10 μF of tantalum electrolytic and 0.01 ICF ceramic.

Real characteristics may differ from various manufacturers and even from the party to the party from one manufacturer. It is important to understand that for efficient work The condenser suppressed by them must be in a lower range than the frequency of their own resonance. Otherwise character reactive resistance It will be inductive, and the condenser will cease to work effectively.

Do not be mistaken about the fact that one 0.1 μF condenser will suppress all frequencies. Small capacitors (10 NF and less) can work more efficiently at higher frequencies.

Power Ins.

The injectation of the power of integrated circuits in order to suppress high-frequency noise is to use one or more capacitors connected between power supply and land. It is important that the conductors connecting condense condensers were short. If this is not the case, then the own inductance of the conductors will play a prominent role and reduce the benefit from the use of unleashing capacitors.

An unleashing capacitor must be connected to each housing of the chip, regardless of how many operating amplifiers are inside the case - 1, 2 or 4. If OU is powered two-polar nutritionIt goes without saying that the disconnecting capacitors should be located for each power output. The tank value must be carefully selected depending on the type of noise and interference present in the diagram.

In particularly difficult cases, the need to add inductance included in series with power output may appear. Inductance should be located before, and not after capacitors.

Another, cheaper way is to replace the inductance resistor with low resistance (10 ... 100 ohms). At the same time, the resistor forms a low-frequency filter along with the disconnecting capacitor. This method reduces the power amplifier power range, which also becomes more dependent on power consumed.

Usually, it is enough to apply one or more aluminum or tantalum electrolytic capacitors to the supply of low-frequency interference in power circuits at the input power connector. An additional ceramic capacitor will suppress high-frequency interference from other boards.

Interchange of input and output signals

Many noise problems are the result of a direct connection of the input and output conclusions. As a result of high-frequency restrictions of passive components, the reaction of the scheme on the impact of high-frequency noise can be sufficiently unpredictable.

In situations where the frequency range of induced noise is largely different from the frequency range of the scheme operation, the solution is simply and obvious - the placement of the passive RC filter to suppress high-frequency interference. However, when applying passive filter It is necessary to be careful: its characteristics (due to the nondealness of frequency characteristics of passive components) lose their properties at frequencies, 100 ... 1000 times larger than the cut frequency (F 3DB). When using sequentially connected filters configured to different frequency ranges, a higher frequency filter must be nearest to a source of interference. Inductance on ferrite rings can also be used to suppress noise; they retain the inductive nature of resistance to some specified frequency, and above their resistance becomes active.

The tipping on the analog scheme can be so large that it is possible to get rid of (or at least reduce) from them by applying screens. For efficient work, they must be carefully designed so that the frequencies that create the greatest problems could not get into the scheme. This means that the screen should not have holes or cuts with dimensions, large than 1/20 wavelength of the shielded radiation. Good idea to divert enough space for an intended screen from the very beginning of the design of the printed circuit board. When using the screen, you can additionally use ferrite rings (or beads) for all connections to the diagram.

Case operating amplifiers

In one case, one, two or four operating amplifiers are usually placed (Fig. 16).

Single OU often also has additional inputsFor example, to adjust the offset voltage. Dual and quad, OU have only inverting and non-inverting inputs and output. Therefore, if necessary, having additional adjustments should be used single operating amplifiers. When using additional conclusions, it is necessary to remember that by its structure they are auxiliary inputs, so the management of them must be accurately accurately and in accordance with the manufacturer's recommendations.

In a single OU, the output is located on the opposite side of the inputs. This can create difficulty when working at high frequencies due to extended feedback conductors. One of the ways to overcome this is to place the amplifier and feedback components on different sides of the printed circuit board. This, however, leads to at least two additional holes and cuts in the land polygon. Sometimes it is worth using a dual OU to resolve this problem, even if the second amplifier is not used (in this case, its conclusions must be connected properly). Figure 17 illustrates a decrease in the length of the feedback circuit conductors for inverting inclusion.

Dual OU are particularly often used in stereo amplifiers, and quadruple - in multi-stage filter diagrams. However, there is a fairly significant minus. Although modern technology Provides a decent isolation between amplifier signals located on one silicon crystal, there are still some cross-interference between them. If it is necessary to have a very small amount of such interference, it is necessary to use single operating amplifiers. Cross interference occurs not only when using dual or quaduble amplifiers. Their source can serve as a very close location of the passive components of different channels.

Dual and quadry OU, besides the foregoing, make it possible to carry out more dense installation. Separate amplifiers, as it were, are mirrored relative to each other (Fig. 18).

Figures 17 and 18 are not shown not all connections required for normal operation, for example, the middle-level formator unipolar nutrition. Figure 19 shows the scheme of such a formator when using the quad amplifier.

The diagram shows all required connections To implement three independent inverting cascades. It is necessary to draw attention to the fact that the conductor conductors of the half of the supply voltage are located directly under the housing of the integrated circuit, which reduces their length. This example illustrates not how it should be, but what should be done. The average voltage, for example, could be one for all four amplifiers. Passive components can be appropriate. For example, the planar components of the size 0402 correspond to the distance between the outputs of the standard SO housing. This allows you to make the length of the conductors very short for high-frequency applications.

When placing operating amplifiers in DIP enclosures and passive components with wire conclusions, the transition holes are required to install them on the printed circuit board. Such components are currently used when there are no special requirements for the size of the printed circuit board; Usually they are cheaper, but the cost of the printed circuit board in the process of manufacture increases due to the drillingings of additional holes under the conclusions of the components.

In addition, when using mounted components, the size of the board and the length of the conductors increase, which does not allow the scheme at high frequencies. Transition holes have their own inductance, which also imposes restrictions on the dynamic characteristics of the scheme. Therefore, attachments are not recommended to be used to implement high-frequency circuits or for analog circuits placed nearby with high-speed logic schemes.

Some developers trying to reduce the length of the conductors, the resistors are vertically. At first glance, it may seem that it reduces the length of the track. However, it increases the path of passing over the resistor, and the resistor itself is a loop (inductance coil). Radiating and receiving ability increases repeatedly.

With surface mounting, no opening is required for each component output. However, problems occur when testing the scheme, and you have to use transition holes as checkpoints, especially when the components of the small size are applied.

Unused Sections OU

When using dual and quadrupid operational amplifiers in the circuit, some of their sections can remain unfounded and must be correctly connected in this case. The erroneous connection can lead to an increase in power consumed, greater heating and greater noise used in the same EU case. Findings of unused operating amplifiers can be connected as shown in Fig. 20a. Connecting outputs with additional components (Fig. 20b) will allow you to easily use this OU when adjusting.

Conclusion

Remember the following highlights and constantly keep them when designing and wiring analog schemes.

General:

think about the printed circuit board as a component electrical circuit
have a presentation and understanding of noise and interference sources
model and squeeze diagrams

Printed circuit board:

use printed circuit boards from quality material (for example, FR-4)
schemes made on multi-layer printed circuit boards, 20 dB less susceptible to external interferences than the diagrams performed on two-layer boards
use separated, non-interpretable polygons for various lands and nutrition
place the land and nutrition polygons on the inner layers of the printed circuit board.

Components:

realize frequency limitations made by passive components and fee
try to avoid vertical location of passive components in high-speed schemes
for high-frequency schemes, use components intended for surface mount
conductors should be the shorter, the better
if a large length of the conductor is required, then reduce its width
unused conclusions of active components must be properly connected.

Wiring:

place an analog schema near the power connector
never divide the conductors transmitting logical signals through an analog board of the board, and vice versa
conductors suitable to the inverting input of OU, make short
make sure that the conductors of the inverting and unconforming inputs of the OU are not located in parallel to each other at a high
try to avoid the use of unnecessary transition holes, because their own inductance can lead to additional problems
do not divide the conductors under the right corners and smooth the tops of the corners, if possible

Interchange:

use the correct types of capacitors to suppress meals in the power circuits
to suppress low-frequency interference and noise, use tantalum capacitors at the input power connector
to suppress high-frequency interference and noise, use ceramic capacitors at the power connector
use ceramic capacitors for each output of the nutrition; If necessary, use several capacitors for different frequency ranges.
if the scheme is excited, then it is necessary to use capacitors with a smaller capacity value, and not large
in difficult cases in power circuits, use successively inclusive resistors of low resistance or inductance
analog output capacitors must be connected only to analog land, and not to digital

Bruce Carter.
OP AMPS FOR EVERYONE, CHAPTER 17
Circuit Board Layout TECHNIQUES
DESIGN REFERENCE, TEXAS INSTRUMENTS, 2002

A little from the theory of antennas

At constant current or low frequencies the active component prevails. With increasing frequency, the reactive component is becoming more and more significant. In the range from 1 kHz to 10 kHz, the inductive component begins to influence, and the conductor is no longer a low-voltage connector, but rather acts as a inductance coil.

The formula for calculating the inductance of the printed circuit board is as follows:

Usually, the tracks on the printed circuit board have values \u200b\u200bfrom 6 NGN to 12 NGN per centimeter of length. For example, a 10-centimeter conductor has a 57-MΩ resistance and an inductance of 8 NGN to see. At a frequency of 100 kHz, reactive resistance becomes 50 mΩ, and at higher frequencies the conductor will be rather inductance than the active resistance.

The pin antenna rule says that it begins to significantly interact with the field at its length about 1/20 from the wavelength, and the maximum interaction occurs at a pin length of 1/4 on the wavelength. Therefore, a 10-centimeter conductor from an example in the previous paragraph will begin to become a pretty good antenna at frequencies above 150 MHz. It is necessary to remember that despite the fact that the digital circuit clock generator may not work at a frequency above 150 MHz, the highest harmonics are always present in its signal. If components with high-length pins are present on the printed circuit board, then such conclusions can also serve as antennas.

The theory of reflection and matching signals is close to the theory of antennas.

There is a capacitive connection between the printed circuit board conductors when they intersect. Sometimes it can create a problem. Conductors who are each other on the adjacent layers create a long film condenser. The capacity of such a capacitor is calculated by the formula shown in Figure 10.

For example, a circuit board may have the following parameters:
- 4 layers; signal and layer of land polygon - adjacent,
- Interlayer interval - 0.2 mm,
- width of the conductor - 0.75 mm,
- Explorer length - 7.5 mm.

The model value of the dielectric constant ER for FR-4 is 4.5.

Substituting all values \u200b\u200bin the formula, we obtain the value of the container between these two tires, equal to 1.1 PF. Even this seemingly small capacity for some applications is unacceptable. Figure 11 illustrates the effect of a container in 1 PF, which occurs when it is connected to the inverting input of the high-frequency operational amplifier.

It can be seen that the amplitude of the output signal is doubted at frequencies close to the upper limit of the frequency range of OU. This, in turn, can lead to generation, especially at the antenna operating frequencies (above 180 MHz).

This effect generates numerous problems to solve which, however, there are many ways. The most obvious of them is to reduce the length of the conductors. Another way is to reduce their width. There is no reason for the use of the conductor of such a width for lining the signal to the inverting input, because By this conductor proceeds very small current. Reducing the length of the track to 2.5 mm, and the widths up to 0.2 mm will lead to a decrease in the tank to 0.1 PF, and such a container will no longer lead to such a significant rise in the frequency response. Another way to solve the problem is the removal of the polygon part under the inverting input and under the conductor suitable for it.

Signal conductors should not be divorced in parallel to each other, except in the case of the wiring of differential or microstrip lines. The gap between the conductors must be at least three times the width of the conductors.

Remember that if there are big resistance in the diagram, then special attention should be paid to clean the board. On the final operations of the manufacture of a printed circuit board should be removed the remains of flux and contamination. Recently, water-soluble fluxes are often used when installing printed circuit boards. Being less harmful, they are easily removed by water. But at the same time, the washing of the board is insufficient water can lead to additional pollution, which worsen the dielectric characteristics. Consequently, it is very important to produce a printed circuit board with a high-impedance circuit with fresh distilled water.

Tie signals

The typical output stage of the logical element contains two transistors, sequentially connected and located between the supply chains and land (Fig. 14).

These transistors in the ideal case work strictly in antiphase, i.e. When one of them is open, then at the same time the second is closed, forming at the output or a logical unit, or a logical zero. In the steady logical state, the power consumption of the logical element is small.

The situation changes dramatically when the output cascade switches from one logical state to another. In this case, for a short period of time, both transistors can be opened at the same time, and the output cascade power current increases, because the resistance of the current path area from the power bus to the land bus is reduced through two successively connected transistors. Power consumed jumps like increases, and then rapidly decreases, which leads to a local change in supply voltage and the occurrence of a sharp, short-term change. Such current changes lead to radiofrequency energy radiation. Even on a relatively simple printed circuit board there can be tens or hundreds of the output cascades of logical elements, so the total effect of their simultaneous work can be very large.

Table 2 presents the maximum operating frequencies for common types of capacitors.

table 2

It is obvious from the table that tantalum electrolytic capacitors are used for frequencies below 1 MHz, ceramic capacitors should be applied at higher frequencies. It must be remembered that the capacitors have their own resonance, and their incorrect choice can not only help, but also aggravate the problem. Figure 15 shows typical own resonances of two overall application capacitors - 10 μF of tantalum electrolytic and 0.01 ICF ceramic.

Real characteristics may differ from various manufacturers and even from the party to the party from one manufacturer. It is important to understand that for the effective operation of the capacitor, the frequencies suppressed them must be in a lower range than the frequency of their own resonance. Otherwise, the nature of reactive resistance will be inductive, and the condenser will cease to work effectively.

Do not be mistaken about the fact that one 0.1 μF capacitor will suppress all frequencies. Small capacitors (10 NF and less) can work more efficiently at higher frequencies.

Power Ins.

The principle of interchanges for the power of integrated circuits in order to suppress high-frequency noise is to use one or more capacitors connected between power supply and land. It is important that the conductors connecting condense condensers were short. If this is not the case, then the own inductance of the conductors will play a prominent role and reduce the benefit from the use of unleashing capacitors.

An unleashing capacitor must be connected to each microcircuit housing, regardless of how many operating amplifiers are inside the case - 1, 2 or 4. If OU is powered by two-polar power, then, of course, the unleashing capacitors should be located for each power output. The value of the container must be carefully selected depending on the type of noise and interference present in the scheme.

In particularly difficult cases, the need to add inductance included in series with power output may appear. Inductance should be located before, and not after capacitors.

Interchange of input and output signals

In situations where the frequency range of induced noise is largely different from the frequency range of the scheme operation, the solution is simply and obvious - the placement of the passive RC filter to suppress high-frequency interference. However, when using a passive filter, it is necessary to be careful: its characteristics (due to the non-idealness of frequency characteristics of passive components) lose their properties at frequencies, 100 ... 1000 times larger than the cut frequency (F 3DB). When using sequentially connected filters configured to different frequency ranges, a higher frequency filter must be nearest to a source of interference. Also, inductance on ferrite rings can be used to suppress noise; They preserve the inductive nature of the resistance to a certain frequency, and above their resistance becomes active.

The tips on the analog scheme can be so large that it is possible to get rid of them (or at least reduce) only by applying screens. For efficient work, they must be carefully designed so that the frequencies that create the greatest problems could not get into the scheme. This means that the screen should not have holes or cuts with dimensions, large than 1/20 wavelength of the shielded radiation. Good idea to divert enough space for an intended screen from the very beginning of the design of the printed circuit board. When using the screen, you can additionally use ferrite rings (or beads) for all connections to the diagram.

Case operating amplifiers

In one case, one, two or four operating amplifiers are usually placed (Fig. 16).

Single OU often also has additional inputs, for example, to adjust the offset voltage. Dual and quad, OU have only inverting and non-inverting inputs and output. Therefore, if necessary, having additional adjustments should be used single operating amplifiers. When using additional conclusions, it is necessary to remember that in its structure they are auxiliary inputs, so the management of them should be accomplished neatly and in accordance with the manufacturer's recommendations.

In a single OU, the output is located on the opposite side of the inputs. This can create difficulties when working in high frequencies due to extended feedback conductors. One of the ways to overcome this is to place the amplifier and feedback components on different sides of the printed circuit board. This, however, leads to the emergence of at least two additional holes and cutouts in the land test site. Sometimes it is worth using a dual OU to resolve this problem, even if the second amplifier is not used (in this case, its conclusions must be connected properly). Figure 17 illustrates a decrease in the length of the feedback circuit conductors for inverting inclusion.

Dual OU are particularly often used in stereo amplifiers, and quadruple - in multi-stage filter diagrams. However, this is quite significant minus. Despite the fact that modern technology provides decent isolation between the amplifier signals located on one silicon crystal, there are still some cross-interference between them. If it is necessary to have a very small amount of such interference, it is necessary to use single operating amplifiers. Cross interference occurs not only when using dual or quaduble amplifiers. Their source can serve as a very close location of the passive components of different channels.

Dual and quadry OU, besides the foregoing, make it possible to carry out more dense installation. Separate amplifiers, as it were, are mirrored relative to each other (Fig. 18).

Figures 17 and 18 are not shown not all connections required for normal operation, for example, the middle-level formator for unipolar nutrition. Figure 19 shows the scheme of such a formator when using the quad amplifier.

The diagram shows all the necessary connections to implement three independent inverting cascades. It is necessary to draw attention to the fact that the conductor conductors of the half of the supply voltage are located directly under the housing of the integrated circuit, which reduces their length. This example illustrates not how connections must be made, but what should be done with the placement of components and tracing. The average voltage, for example, could be one for all four amplifiers. Passive components can be appropriate. For example, the planar components of the size 0402 correspond to the distance between the outputs of the standard SO housing. This allows the length of the conductors for high-frequency applications is very short.

Volumetric and surface assembly

In addition, when using mounted components, the size of the board and the length of the conductors increase, which does not allow the scheme to operate at high frequencies. Transition holes have their own inductance, which also imposes restrictions on the dynamic characteristics of the scheme. Therefore, attachments are not recommended to be used to implement high-frequency circuits or for analog circuits placed near high-speed logic schemes.

Unused Sections OU

Conclusion

Remember the following highlights and constantly keep them when designing and wiring analog schemes.

General:

Think about the printed circuit board as a component of the electrical circuit;
. Have a presentation and understanding of noise and interference sources;
. Model and squeeze diagrams.

Printed circuit board:

Use printed circuit boards from high-quality material (for example, FR-4);
. Schemes made on multi-layer printed circuit boards, 20 dB are less susceptible to external interference than the diagrams performed on two-layer boards;
. Use separated, non-varying polygons for various lands and nutrition;
. Place the land and nutrition polygons on the inner layers of the printed circuit board.

Components:

Realize the frequency limitations made by passive components and fee of the board;
. Try to avoid vertical location of passive components in high-speed schemes;
. For high-frequency schemes, use components intended for surface mounting;
. Conductors should be the shorter, the better;
. If a high length of the conductor is required, then reduce its width;
. Unused conclusions of active components must be properly connected.

Wiring:

Place an analog schema near the power connector;
. Never divide the conductors passing logical signals through an analog board area, and vice versa;
. Conductors suitable for the inverting input of OU, make short;
. Make sure that the conductors of the inverting and non-inverting inputs of the OS are not located in parallel to each other at a high outrest;
. Try to avoid the use of unnecessary transition holes, because their own inductance can lead to additional problems;
. Do not divide the conductors under right corners and smooth the tops of the corners, if possible.

Interchange:

Use the correct types of capacitors to suppress interference in the power circuits;
. To suppress low-frequency interference and noise, use tantalum capacitors at the input power connector;
. To suppress high-frequency interference and noise, use ceramic capacitors at the input power connector;
. Use ceramic capacitors for each output of the nutrition; If necessary, use several capacitors for different frequency ranges;
. If an excitement occurs in the scheme, it is necessary to use capacitors with a lower capacitance value, and not large;
. In difficult cases in power circuits, use successively included resistors of low resistance or inductance;
. An analog power condensers must be connected only to analog land, and not to digital.

Bruce Carter.
OP AMPS FOR EVERYONE, CHAPTER 17
Circuit Board Layout TECHNIQUES
DESIGN REFERENCE, TEXAS INSTRUMENTS, 2002

Thank you website elarart.narod.ru for the transferred translation

"Iron-laser" technology of manufacturing printed circuit boards (ULT) Literally in a couple of years, it spread widely in the amateurs of amateur circles and allows you to receive printed circuit boards high Quality. Pubboard cards with "manual drawing" require high time and are not insured against errors.

Special requirements for the accuracy of the picture are presented in the manufacture of printed inductors for high-frequency chains. The edges of the coil conductors must be as smooth as possible, as it affects their quality. Manually draw a picture of a multi-ski spiral coil is very problematic, and here the ULT may well say "his word".

Fig. one

Fig. 2.

So, everything is in order. Run computer program Sprint-layout, for example, version 5.0. Install in the program settings:

Scale of the coordinate grid - 1.25 mm;

Line width - 0.8 mm;

Board sizes - 42.5x42.5 mm;

External diameter "Piglet" - 1.5 mm;

The diameter of the hole in the "Piglet" - 0.5 mm.

We find the center of the board and draw a coil conductor template (Fig. 1)according to the coordinate grid using the Explorer tool, spinning the coil in the desired side (a mirror image is necessary for the template, but it can be obtained later when printing). At the beginning and to the end of the coil, we install the "Piglet" to connect the coil with the elements of the circuit.

In the settings for printing, we set the number of prints on the sheet, the distance between the prints and, if you want to "spin" the coil to the other side, the mirror print of the pattern. You should print on smooth paper or a special film by installing the maximum toner supply in the printer settings when printing.

Next, follow the standard ULT. We prepare the foil fiberglass, we clean the surface of the foil and degrease, for example, acetone. We apply a toner pattern to foil and stroke a hot iron through a sheet of paper to a reliable toner clutch with a foil.

After under the jet of water from under the tap (cold or room temperature), we discharge the paper and carefully "katoshi" remove it, leaving the toner on the foil board. We produce platform etching and subsequent removal of toner with solvent, for example, acetone. The board remains a clear conductor of the "printed" inductance of high quality inductors.

Print coils with spiral coils on the ULT are a little worse quality. The point here is in square form of pixels of the image, so the edges of the conductor of the spiral coil are obtained by the toothed. True, these irregularities are small enough, and the quality of the coil, in general, is still higher than with manual execution.

Open the Sprint-Layout version 5.0 again. In the toolkit, choose Special Form - a tool for drawing polygons and spirals. Select the Spiral tab. Install:

Initial radius (START RADIUS) -2 mm;

Distance between Vitka (Distance) -1.5 mm;

Explorer width (TRACK WIDTH) -0.8 mm;

Number of turns (TURNS), for example, - 20.

The size of a fee occupied by such a coil is 65x65 mm (Fig. 2).

Print coils are usually associated with each other strip filters (PF) with low-capacity capacitors. However, their inductive relationship is also possible, the degree of which can be changed by changing the distance between the coil planes or eccentric turning one relative to the other. Fixed mounting coils relative to each other can be

average dielectric struts.

The inductance of the coil inductors can be closed with turns, breaking the printed conductor or its partial removal. This will increase the frequency of the circuit setting. Frequency reduction can be achieved by soldering the capacitors of the small capacity of SMD types between the turns.

Production of coils of the VHF range in the form of meander, straight and curved lines, comb filters, etc. UHA also adds to the final product of the grace and, as a rule, increases their quality facilities (due to the "smooth" edges of printed conductors). However, in the manufacture, it should be remembered for the quality of the substrate material (fiberglass), which with increasing frequency loses its properties of the insulator. In equivalent circuits, the resistance of the losses in the dielectric should be included in parallel printing coils, and this resistance will be the less than the above the operating frequency and worse than the dielectric quality.

In practice, folgized fiberglass can be fully used for the manufacture of printed resonant chains up to a 2-meter range inclusive (about 150 MHz). Special high-frequency glass grade glassstolite can be used in the range of 70 cm (approximately 470 ... 500 MHz). At higher frequencies, a foil RF fluoroplast (Teflon), ceramics or glass should be applied.

The inductor's printed coil has an increased voluntary due to the reduction of the gravy container obtained by, on the one hand, due to the low thickness of the foil, on the other, the "winding" steps. The closed frame from the grounded foil around the printed coil in its plane serves as a screen from other coils and printed conductors, but little affects the parameters of the coil if its peripherals are under small RF voltage (connected to the shared wire), and the center is high.

Literature

1. G. Panasenko. Production of printed coils. - Radio, 1987, №5, p.62.

The intention of this article is to discuss common mistakes performed by printed circuit board developers, a description of the impact of these errors to qualitative indicators and recommendations for resolving problems that have arisen.

General considerations

Due to the essential differences of the analog circuit engineering from the digital, the analog part of the circuit should be separated from the rest, and when it is wiring, special methods and rules must be respected. Effects resulting from non-ideal characteristics of printed circuit boards are becoming particularly noticeable in high-frequency analog schemes, but the errors of the general form described in this article can affect the qualitative characteristics of devices that are working even in the sound frequency range.

Printed circuit board - component of the scheme

Only in rare cases, the printed circuit board of the analogue scheme can be divorced so that the impacts made by it have no effect on the operation of the scheme. At the same time, any such effect can be minimized so that the characteristics of the analog scheme of the device were the same as the characteristics of the model and the prototype.

Maketing

Digital circuit developers can adjust small errors on the manufactured board, complementing it with jumpers or, on the contrary, removing unnecessary conductors, making changes to the operation of programmable chips, etc., moving very soon to the next development. For an analog schema, the situation is wrong. Some of the common errors discussed in this article cannot be corrected by adding jumpers or removing unnecessary conductors. They can and will put the entire printed circuit board in the entire state.

It is very important for the developer of digital diagrams that uses such correction methods, read and understand the material set out in this article, in advance before the transfer of the project to production. Some attention paid to development and discussion possible options It will help not only prevent the printed circuit board in the leaning, but also reduce the cost due to gross errors in a small analog part of the scheme. Error search and their correction can lead to loss of hundreds of hours. Maketing can shorten this time to one day or less. Make all your analog schemes.

Sources of noise and interference

Noise and interference are basic elements that limit the qualitative characteristics of the schemes. Interference can be reduced by sources, so and look at the elements of the scheme. The analog circuit is often located on the printed circuit board along with high-speed digital components, including digital processors (DSP).

High-frequency logical signals create significant radio frequency interference (RFI). The number of noise emission sources is huge: key digital system power sources, cell phones, radio and television, power supply of daylight lamps, personal computers, thunderstorm discharges, etc. Even if the analog scheme works in the sound frequency range, radio frequency interference can create noticeable noise in the output signal.

Categories of printed circuit board

Selection of a printed circuit board is an important factor determining mechanical characteristics When using the device as a whole. For the manufacture of printed circuit boards, materials are used in various levels of quality. The most suitable and convenient for the developer will be if the manufacturer of printed circuit boards is nearby. In this case, it is easy to control the specific resistance and the dielectric constant - the main parameters of the printed circuit board. Unfortunately, this is not enough and often the knowledge of other parameters, such as flammability, high-temperature stability and hygroscopicity coefficient. These parameters can know only the manufacturer of components used in the production of printed circuit boards.

The layered materials are indicated by FR (FLAME Resistant, resistance to ignition) and G. The material with the FR-1 index has the greatest flammability, and the FR-5 is the smallest. Materials with G10 and G11 indexes have special characteristics. Printing materials are shown in Table. one.

Do not use the FR-1 printed circuit board. There are many examples of using FR-1 printed circuit boards, on which there are damage from the thermal effects of powerful components. Printed circuit boards of this category are more similar to cardboard.

FR-4 is often used in the manufacture of industrial equipment, while FR-2 is used in production household appliances. These two categories are standardized in industry, and FR-2 and FR-4 printed circuit boards are often suitable for most applications. But sometimes the imperfection of the characteristics of these categories makes it use other materials. For example, for very high-frequency applications, fluoroplast and even ceramics are used as a material of printed circuit boards. However, the more exotic material of the printed circuit board, the higher the price can be.

When choosing a printed circuit board, pay special attention to its hygroscopicity, since this parameter can have a strong negative effect on the desired fee characteristics - surface resistance, leakage, high-voltage insulating properties (samples and sparks) and mechanical strength. Also pay attention to operating temperature. Sections with high temperatures can occur in unexpected places, for example, next to large digital integral circuits, whose switching occurs at high frequency. If such sites are located directly under analog components, the temperature rise may affect the change in the characteristics of the analog scheme.

Table 1

	Components, comments
	paper, phenolic composition: pressing and stamping at room temperature, high hygroscopicity coefficient
	paper, phenolic composition: applicable for unilateral printed circuit boards of household appliances, low hygroscopicity coefficient
	paper, epoxy composition: Developments with good mechanical and electrical characteristics
	fiberglass, epoxy composition: excellent mechanical and electrical properties
	fiberglass, epoxy composition: High strength at elevated temperatures, no ignition
	fiberglass, epoxy composition: High insulating properties, the highest strength of fiberglass, low hygroscopicity coefficient
	fiberglass, epoxy composition: High bend strength at elevated temperatures, high resistance solvents

After the printed board is selected, it is necessary to determine the foil thickness of the printed circuit board. This parameter is primarily selected based on the maximum value of the flowing current. If possible, try to avoid the use of very thin foil.

Number of layers of printed circuit board

Depending on the total complexity of the scheme and qualitative requirements, the developer must determine the number of printed circuit board.

Single-layer printed circuit board

Very simple electronic circuits are performed on unilateral boards using cheap foil materials (FR-1 or FR-2) and often have many jumpers, reminding double-sided fees. This method of creating printed circuit boards is recommended only for low-frequency schemes. For reasons to be described below, one-sided printed circuit boards are largely susceptible to the tip. A good one-sided printed circuit board is quite difficult to develop because of many reasons. However good fees This type is found, but when they are developing, it takes a lot of thinking in advance.

Two-layer printed fees

At the next level there are two-way printed circuit boards, which in most cases are used as the FR-4 substrate material, although FR-2 is sometimes found. The use of FR-4 is more preferable because in printed circuit boards from this material, the holes are obtained more better quality. Schemes on bilateral printed circuit boards are dissolved much easier, because In two layers it is easier to carry out the wiring of intersecting trails. However, for analog circuits, the crossing of the tracks is not recommended. Where possible, the bottom layer (Bottom) must be diverted under the land polygon, and the remaining signals are breeding in the upper layer (TOP). The use of a landfill as an earthen tire gives several advantages:

the total wire is the most frequently connected in the wire scheme; Therefore, it is reasonable to have a lot of overall wire to simplify the wiring.
the mechanical strength of the board increases.
resistance to all connections to the general wire decreases, which, in turn, reduces noise and tip.
the distributed container increases for each circuit of the scheme, helping to suppress the emitted noise.
the polygon, which is the screen, suppresses the floods emitted by the sources located on the side of the polygon.

Double-sided printed circuit boards, despite all its advantages, are not the best, especially for non-signal or high-speed schemes. In general, the thickness of the printed circuit board, i.e. The distance between the metallization layers is 1.5 mm equals, which is too much for the complete implementation of some advantages of the two-layer printed circuit board, which are above. Distributed capacity, for example, too small due to such a large interval.

Multilayer printed circuit boards

For responsible schemechnical developments require multilayer printed circuit boards (MPP). Some reasons for their use are obvious:

the same convenient, as well as for a common wire tire, power tire layout; If the power tires use polygons on a separate layer, it is quite simple with the help of transition holes to carry out a power supply to each element of the circuit;
signal layers are exempt from power tires, which makes it easier for signaling warning;
a distributed container appears between land and nutrition polygons, which reduces high-frequency noise.

In addition to these reasons for the application of multilayer printed circuit boards, there are other, less obvious:

best suppression of electromagnetic (EMI) and radio frequency (RFI) interference due to the effect of reflection (Image Plane Effect), known during the time of Marconi. When the conductor is placed close to a flat conductive surface, most of the return high-frequency currents will flow through the plane directly under the conductor. The direction of these currents will be opposite to the direction of currents in the conductor. Thus, the reflection of the conductor in the plane creates a signal transmission line. Since currents in the conductor and in the plane are equal in size and are opposite to the direction, a slight decrease in the emitted interference is created. The reflection effect effectively works only with inseparable solid polygons (they can be both land polygons and power polygons). Any integrity impairment will lead to a decrease in interference suppression.
Reducing the total value with small-scale production. Despite the fact that the manufacture of multilayer printed circuit boards is more expensive, their possible radiation is less than that of single and two-layer boards. Consequently, in some cases, the use of only multilayer boards will allow the requirements for radiation set in the development, and do not conduct additional tests and testing. The use of the MPP can reduce the level of radiated interference by 20 dB compared to two-layers.

The order of the layers

For printed circuit boards with more than four layers, there is a general rule to have high-speed signal conductors between land and power polygons, and low-frequency to remove external layers.

Ground

Good grounding is a general requirement of a saturated, multi-level system. And it should be planned from the first step of designer development.

Basic rule: Separation of the Earth.

Other rules for the formation of the Earth:

Tires for food and land must be under one variable current potential, which implies the use of condensers of the junction and distributed container.
Do not allow overlaps of analog and digital polygons. Place the tires and polygons of analog power over the landfill of the analog land (similar to the digital power tires). If in any place there is an overlap of an analog and digital polygon, the distributed capacity between overlapping sites will create a change over a variable current, and the tip of the digital components will fall into an analog circuit. Such overlaps annul the insulation of the polygons.
The separation does not mean electrical insulation analogue from digital land. They must be connected together in some, preferably one, low-impedance node. Correct, from the point of view of the Earth, the system has only one ground, which is a grounding output for systems with power supply voltage or a common output for power supply systems constant voltage (for example, battery). All signaling currents and power currents in this scheme should be returned to this land in one point, which will serve as systemic land. Such a point may be the output of the device body. It is important to understand that when connecting the general output of the circuit to several points of the case, earth contours can form. The creation of the only total land population point is one of the most difficult aspects of systemic design.
If possible, share the connectors conclusions designed to transmit return currents - return currents should be combined only at the point of system earth. The aging of the contacts of the connectors, as well as the frequent dominant of their response parts leads to an increase in the resistance of the contacts, therefore, for more reliable operation, it is necessary to use connectors with a certain number of additional conclusions. Complex digital circuit boards have many layers and contain hundreds or thousands of conductors. Adding another conductor rarely creates a problem in contrast to the additional terminals of the connectors. If this fails to do, then you need to create two return current conductor for each power chain on the board, observing special precautions.
It is important to separate tires digital signals From the locations on the printed circuit board, where the analog circuit components are located. This implies insulation (shielding) by polygons, creating short tracks of analog signals and attentive placement of passive components in the presence of a number of high-speed digital and responsible bus signals. Tires of digital signals should be divorced around areas with analog components and do not overlap with tires and polygons of analog land and analog power. If this is not done, the development will contain a new unforeseen element - antenna, the radiation of which will affect high-impedance analog components and conductors.

The titles of the conclusions (analog or digital) refer only to the internal structure of the converter, to its internal connections. In the diagram, these conclusions must be connected to the analog land bus. The compound can be performed within the integrated circuit, but it is quite difficult to obtain a low resistance of such a compound due to topological restrictions. Therefore, when using converters, an external compound of the findings of the analog and digital land is assumed. If this is not done, then the parameters of the chip will be much worse than the specification.

It is necessary to take into account the fact that the digital elements of the converter may worsen the qualitative characteristics of the scheme, bringing digital interference in the chain of the analog land and analog power. When developing converters, this negative impact is taken into account so that the digital part consuming as little power as possible. In this case, interference from switching logical elements decreases. If the digital converter squeezes are not much loaded, the internal switching usually do not cause special problems. When developing a printed circuit board containing a ADC or DAC, it is necessary to properly refer to the disconnection of the digital power supply of the converter to the analog land.

Frequency characteristics of passive components

For the correct operation of the analog schemes, the right choice of passive components is very important. Start designer development with careful consideration of the high-frequency characteristics of passive components and pre-location and layouts on the sketch of the board.

A large number of developers completely ignore frequency limits of passive components when used in analog circuit engineering. These components have limited frequency bands and their work outside the specified frequency domain can lead to unpredictable results. Someone may think that this discussion concerns only high-speed analog schemes. However, this is far from the way - high-frequency signals strongly affect the passive components of low-frequency circuits by radiation or direct communication on the conductors. For example, a simple low-frequency filter on an operating amplifier can easily turn into a high-frequency filter when exposed to its high frequency input.

Resistors

Usually apply resistors of three types: 1) wire, 2) carbon composite and 3) film. It is not necessary to have a lot of imagination in order to understand how a wire resistor can turn into inductance, since it is a coil with a wire of high-wing metal. Most developers electronic devices Do not have the concept of the internal structure of film resistors, which are also a coil, however, from a metal film. Therefore, film resistors also have inductance, which is less than that of wire resistors. Film resistors with resistance of no more than 2 kΩ can be freely used in high-frequency schemes. The conclusions of the resistors are parallel to each other, therefore there is a noticeable capacitive connection between them. For resistors with greater resistance, the inter-exterior container will reduce the full impedance at high frequencies.

Condencators

The high-frequency characteristics of the capacitors can be represented by the equivalent circuit shown in Figure 6.

Capacitors in analog circuits are used as elements of the junction and filter components.

The electrolytic capacitor with a capacity of 10 μF has 1.6 ohm resistance at a frequency of 10 kHz and 160 μC at a frequency of 100 MHz. Is it so?

When using electrolytic capacitors, you must monitor the correct connection. A positive output must be connected to a more positive constant potential. Incorrect connection leads to a flow through an electrolytic constant current capacitor, which may not only be due to the condenser itself, but also part of the scheme.

Inductance

Printed circuit board

The printed fee itself has the characteristics of the passive components discussed above, however, not so obvious.

A little from the theory of antennas

The pin antenna rule says that it begins to significantly interact with the field at its length about 1/20 from the wavelength, and the maximum interaction occurs at a pin length of 1/4 on the wavelength. Therefore, a 10-centimeter conductor from an example in the previous paragraph will begin to become a pretty good antenna at frequencies above 150 MHz. It is necessary to remember that despite the fact that the digital circuit clock generator may not work at a frequency above 150 MHz, the highest harmonics are always present in its signal. If components with high-length pins are present on the printed circuit board, then such conclusions can also serve as antennas.

The theory of reflection and matching signals is close to the theory of antennas.

For example, a circuit board may have the following parameters:
- 4 layers; signal and layer of land polygon - adjacent,
- Interlayer interval - 0.2 mm,
- width of the conductor - 0.75 mm,
- Explorer length - 7.5 mm.

The model value of the dielectric constant ER for FR-4 is 4.5.

The value of the tank between the two tires is 1.1 PF. Even this seemingly small capacity for some applications is unacceptable.

The amplitude of the output signal at frequencies close to the upper limit of the frequency range of OU is doubted. This, in turn, can lead to generation, especially at the antenna operating frequencies (above 180 MHz).

The width of the printed circuit board is impossible to reduce infinitely. The maximum width is defined both by the process and foil thickness. If two conductors take close to each other, then the capacitive and inductive communication is formed between them.

For example, with d \u003d 0.4 mm and H \u003d 1.5 mm (sufficiently common values) the inductance of the opening is 1.1 NGN.

Remember that if there are big resistance in the diagram, then special attention should be paid to clean the board. On the final operations of the manufacture of a printed circuit board should be removed the remains of flux and contamination. Recently, water-soluble fluxes are often used when installing printed circuit boards. Being less harmful, they are easily removed by water. But at the same time, the washing of the board is insufficient water can lead to additional pollution, which worsen the dielectric characteristics. Consequently, it is very important to produce a printed circuit board with a high-impedance circuit with fresh distilled water.

Tie signals

The typical output stage of the logical element contains two transistors, sequentially interconnected, as well as between the supply chains and land.

The situation changes dramatically when the output cascade switches from one logical state to another. In this case, for a short period of time, both transistors can be opened at the same time, and the output cascade power current increases, because the resistance of the current path area from the power bus is reduced to the land bus through two successively connected transistors. Power consumed jumps like increases, and then decreases, which leads to a local change in supply voltage and the occurrence of a sharp, short-term change. Such current changes lead to radiofrequency energy radiation. Even on a relatively simple printed circuit board, there may be dozens or hundreds of considered output cascades of logical elements, so the total effect of their simultaneous work can be very large.

Table 2 presents the maximum operating frequencies for common types of capacitors.

table 2

Real characteristics may differ from various manufacturers and even from the party to the party from one manufacturer. It is important to understand that for the effective operation of the capacitor, the frequencies suppressed them must be in a lower range than the frequency of their own resonance. Otherwise, the nature of reactive resistance will be inductive, and the condenser will cease to work effectively.

Do not be mistaken about the fact that one 0.1 μF capacitor will suppress all frequencies. Small capacitors (10 NF and less) can work more efficiently at higher frequencies.

Power Ins.

An unleashing capacitor must be connected to each microcircuit housing, regardless of how many operating amplifiers are inside the case - 1, 2 or 4. If OU is powered by two-polar power, then, of course, the unleashing capacitors should be located for each power output. The tank value must be carefully selected depending on the type of noise and interference present in the diagram.

In particularly difficult cases, the need to add inductance included in series with power output may appear. Inductance should be located before, and not after capacitors.

Interchange of input and output signals

In situations where the frequency range of induced noise is largely different from the frequency range of the scheme operation, the solution is simply and obvious - the placement of the passive RC filter to suppress high-frequency interference. However, when using a passive filter, it is necessary to be careful: its characteristics (due to the non-idealness of frequency characteristics of passive components) lose their properties at frequencies, 100 ... 1000 times larger than the cut frequency (F3DB). When using sequentially connected filters configured to different frequency ranges, a higher frequency filter must be nearest to a source of interference. Inductance on ferrite rings can also be used to suppress noise; They preserve the inductive nature of the resistance to a certain frequency, and above their resistance becomes active.

Case operating amplifiers

One, two or four operating amplifiers are usually placed in one case.

Single OU often also has additional inputs, for example, to adjust the offset voltage. Dual and quad, OU have only inverting and non-inverting inputs and output. Therefore, if necessary, having additional adjustments should be used single operating amplifiers. When using additional conclusions, it is necessary to remember that by its structure they are auxiliary inputs, so the management of them must be accurately accurately and in accordance with the manufacturer's recommendations.

Dual OU are particularly often used in stereo amplifiers, and quadruple - in multi-stage filter diagrams. However, there is a fairly significant minus. Despite the fact that modern technology provides decent isolation between the amplifier signals located on one silicon crystal, there are still some cross-interference between them. If it is necessary to have a very small amount of such interference, it is necessary to use single operating amplifiers. Cross interference occurs not only when using dual or quaduble amplifiers. Their source can serve as a very close location of the passive components of different channels.

Dual and quadry OU, besides the foregoing, make it possible to carry out more dense installation. Separate amplifiers seem to be mirrored relative to each other.
It is necessary to draw attention to the fact that the conductor conductors of the half of the supply voltage are located directly under the housing of the integrated circuit, which reduces their length. This example illustrates not how it should be, but what should be done. The average voltage, for example, could be one for all four amplifiers. Passive components can be appropriate. For example, the planar components of the size 0402 correspond to the distance between the outputs of the standard SO housing. This allows you to make the length of the conductors very short for high-frequency applications.

Unused Sections OU

When using dual and quadrupid operational amplifiers in the circuit, some of their sections can remain unfounded and must be correctly connected in this case. The erroneous connection can lead to an increase in power consumed, greater heating and greater noise used in the same EU case. Findings of unused operating amplifiers can be connected as: the output of the amplifier is connected to the inverting input.

Conclusion

Remember the following highlights and constantly keep them when designing and wiring analog schemes.

think about the printed circuit board as a component of the electrical circuit;
have a presentation and understanding of noise and interference sources;
model and squeeze diagrams.

Printed circuit board:

use printed circuit boards from high-quality material (for example, FR-4);
schemes made on multi-layer printed circuit boards, 20 dB less susceptible to external interference than the diagrams performed on two-layer boards;
use separated, non-varying polygons for various lands and nutrition;
place the land and nutrition polygons on the inner layers of the printed circuit board.

Components:

realize the frequency limitations made by passive components and fee of the board;
try to avoid vertical location of passive components in high-speed schemes;
for high-frequency schemes, use components intended for surface mounting;
conductors should be the shorter, the better;
if a high length of the conductor is required, then reduce its width;
unused conclusions of active components must be properly connected.

Wiring:

place an analog schema near the power connector;
never divide the conductors passing logical signals through an analog board area, and vice versa;
conductors suitable for the inverting input of OU, make short;
make sure that the conductors of the inverting and non-inverting inputs of the OS are not located in parallel to each other at a high outrest;
try to avoid the use of unnecessary transition holes, because their own inductance can lead to additional problems;
do not divide the conductors under right corners and smooth the tops of the corners, if possible.

Interchange:

use the correct types of capacitors to suppress interference in the power circuits;
to suppress low-frequency interference and noise, use tantalum capacitors at the input power connector;
to suppress high-frequency interference and noise, use ceramic capacitors at the input power connector;
use ceramic capacitors for each output of the nutrition; If necessary, use several capacitors for different frequency ranges;
if an excitement occurs in the scheme, it is necessary to use capacitors with a lower capacitance value, and not large;
in difficult cases in power circuits, use successively included resistors of low resistance or inductance;
an analog power condensers must be connected only to analog land, and not to digital.