Soft and hard modes of self-excitation of the oscillator. Oscillator self-excitation modes. Summary: Basic schemes of self-excitation modes

Let's go back to fig. 9.6 and find out the behavior of the oscillator when the feedback coefficient changes. When the coupling is weakened, the slope of line II increases, and at which critical value, which turns inequality (9.13) into equality, the occurrence of oscillations is impossible. The communication line corresponding to the critical feedback takes the position of OB.

If in an oscillator with inductive feedback and an oscillatory characteristic shown in Fig. 9.6, increase M smoothly, then starting from the critical value, the amplitude of the stationary oscillation will smoothly increase, as shown in Fig. 9.8. This self-excitation mode is called soft. It follows from what has been said that to obtain a soft mode, it is necessary that the oscillatory characteristic leave the zero point and have a sufficiently large slope in the region of small amplitudes. All of these requirements are met when using automatic offset.

When using forced (external) displacement, the vibrational characteristic takes the form shown in Fig. 9.9. For oscillations to occur in this case, a very strong feedback (line, mutual induction) is required.

Rice. 9.8. Dependence of the stationary amplitude on feedback in the soft mode

Rice. 9.9. Vibration characteristic corresponding to a hard mode

Rice. 9.10. Dependence of the stationary amplitude on feedback in the rigid mode

Rice. 9.11. On the question of the stability of generation in a hard mode

After the vibrations are established, the connection can be weakened to the value at which the communication line takes the position of the OB. With further weakening of the connection, the oscillations are broken. To restore oscillations, M must be increased to the value of the corresponding communication line OA. This self-excitation mode is called

The dependence of the stationary amplitude on M in the hard mode is shown in Fig. 9.10, and arrows indicate the direction of change of M.

If the forced bias voltage is so large that the oscillatory response does not start from zero (Figure 9.11), then no increase in feedback can cause self-oscillation. If, however, vibrations are caused with the help of an external influence, then with a sufficiently strong feedback, fluctuations can exist even after the cessation of the influence. Of the two intersection points of lines I and II, point C is stable, and point D is unstable (we mean dynamic stability, i.e., generation stability). This means that with small random deviations of the amplitude of the current in the circuit near point C, the system returns to its original state, however small deviation of the amplitude in the region of point D progressively increases and transfers the amplitude either to a stable point C or to point 0 (corresponding to static stability ). The proof of the instability of the point D is similar to the proof of the stability of the point C given in the previous section.

If in an autogenerator with inductive feedback and an oscillatory characteristic, M is gradually increased, then, starting from the critical value of M cr, the amplitude of the stationary oscillation will smoothly increase.

This self-excitation mode is called light.

To obtain a light mode, it is necessary that the oscillatory characteristic leave the zero point and have a sufficiently large slope in the region of small amplitudes. All of these requirements are met when using automatic offset. When using forced (external) displacement, the vibrational characteristic takes the form:

For the occurrence of oscillations in this case, a very strong feedback is required (line OA, mutual induction M 1).

After the vibrations have been established, the connection can be weakened to the value of M 2, at which the communication line takes the position of the OB. With further weakening of the connection, the oscillations break down. To restore the oscillations of M, corresponding to the communication line OA. This self-excitation mode is called hard.

Purpose, classification and principles of construction of synchronization systems.

In most cases, the normal functioning of various information transmission systems requires a certain synchronization of the operation of the transmitting and receiving equipment. This function is usually assigned to special synchronization systems. Their noise immunity and the quality of the transmission system as a whole depend on their noise immunity and the quality of their work. Synchronization systems form on the receiving side special synchronizing signals, synchronous with the corresponding signals generated on the transmitting side, taking into account the distortions that appeared during the propagation of signals through the transmission channel.

All the variety of tasks facing synchronization systems can be divided into two large classes: synchronization of various types of switching devices in order to ensure time separation of signals (in systems with time division of channels), synchronization of the operation of receiving and processing devices in order to increase their noise immunity (when receiving signals with random parameters).

Real transmission channels are variable parameters.

Optimal reception of signals with random parameters requires evaluation (measurement) of essential parameters (frequency, delay time, phase) of such signals. These measurements are assigned to the synchronization systems.

Synchronization systems are classified according to various criteria. All practical tasks of synchronization in transmission systems can be provided by three synchronization systems: high-frequency, element-wise (clock), group.

The problem of high-frequency synchronization usually arises when using pre-detector correlation signal processing. In this case, at the receiving point, it is necessary to obtain samples of high-frequency signals, the frequencies of which at any time must be equal or close to the frequencies of the carriers or subcarriers of the received signals. In the case of coherent processing, this equality must be satisfied with phase accuracy.

The task of the element-by-element (clock) synchronization is to ensure on the receiving side the fixation of the time boundaries of the chips corresponding to the smallest time interval to be fixed, formed on the transmitting side. The formation of such signals may be necessary to ensure optimal after detector signal processing and separation of signals into their channels.

In analog transmission systems, such chips are usually timeslots (time slots allocated for transmission over one channel), and in digital systems, elementary information symbols.

Group synchronization should be able to capture the timing of certain groups, chips such as words, frames, frames, etc.

In some systems, all three of these types of subsystems can operate simultaneously.

High-frequency I&C sync signals are typically periodic in structure. Group sync signals can be either periodic or form a random stream. In digital transmission systems with cyclic and periodic polling, when all three indicated types of synchronization can operate, the frequencies of all the listed types of synchronization can be selected as multiples of each other.

For example, each frame (group of bursts) contains n 1 words, each word consists of n 2 symbols, and each symbol lasts only n 3 periods of the high frequency carrier or subcarrier. In this case, all types of synchronization can be performed after setting the frame synchronization.

Depending on the values ​​of constant supply voltages supplied to the electrodes of the amplifying element, and on the coefficient K 0. c two modes of self-excitation are possible: soft and hard.

In the soft self-excitation mode, the operating point A is selected on the linear section of the I - V characteristic of the amplifying element (Figure 9.1, a), which provides the initial operating mode of the amplifying element without cutting off the output current. Under these conditions, self-excitation arises from the smallest changes in the input voltage, which are always present in real conditions due to fluctuations of charge carriers.

At first, oscillations in the oscillator build up relatively quickly. Then, due to the nonlinearity of the I - V characteristic of the amplifying element, the growth of the oscillation amplitude slows down, since the voltage at its input falls on the sections of the I - V characteristic with an ever smaller static slope, and this leads to a decrease in the average slope S Wed and transmission coefficient K 0s feedback loops.

Figure 9.1 - Diagrams explaining self-excitation modes.

The increase in vibrations occurs as long as the transmission coefficient decreases to unity. As a result, a stationary mode will be established in the oscillator, which corresponds to a certain amplitude of the output oscillations, and the cutoff angle of the output current is 0> 90 °. The frequency of these vibrations is very close to the resonant frequency of the vibrating system. Pay attention: if the amplifying element had a linear current-voltage characteristic, the amplitude of self-oscillations would grow to infinity, which is physically impossible. Therefore, it is impossible to obtain stable self-oscillations with a constant amplitude in a linear circuit.

Due to the nonlinearity of the current-voltage characteristic, the shape of the output current of the amplifying element is non-sinusoidal. However, with a sufficiently high figure of merit (Q = 50 ... 200) of the oscillating system, the first harmonic of this current and, therefore, the voltage at the output of the oscillator are almost harmonic oscillations.

9.5 Hard self-excitation mode

In this mode, the bias voltage is set so that at low amplitudes of the input voltage, the current does not pass through the amplifying element. Then, minor fluctuations in the circuit cannot cause a current in the output circuit, and self-excitation of the oscillator does not occur. Oscillations arise only when their initial amplitude is sufficiently large, which cannot always be ensured. The process of emergence and growth of oscillations in a hard mode of self-excitation is illustrated in Figure 9.1, b. It can be seen that at small initial amplitudes of the input voltage (curve 1), the current i out = 0 and self-oscillations do not arise. They arise only at a sufficiently large initial voltage amplitude (curve 2) and rapidly increase to a steady-state value. In stationary mode, the amplifying element operates with the cutoff angles of the output current<90°.

For the convenience of operating the autogenerator, it is more expedient to use a soft self-excitation mode, since in this mode, oscillations arise immediately after the power source is turned on. However, in a rigid vibration mode with a cutoff angle<90° обеспечиваются более высокий КПД автогенератора и меньшие тепловые потери. Поэтому в стационарном режи­ме автогенератора более выгоден имен­но режим с малыми углами отсеч­ки выходного тока усилительного эле­мента.

SUSTAINABILITY OF AG'S WORK

It is convenient to investigate the process of occurrence and establishment of oscillations in an oscillator using oscillatory characteristics and feedback lines.

10.1 Vibrational characteristics

They represent the dependences of the amplitude of the first harmonic of the output current of the amplifying element I m 1 on the amplitude of the input voltage U m in at constant bias voltage U 0 and open loop feedback:. These dependences are nonlinear and can be obtained experimentally by switching the generator to the mode with external excitation.

Figure 10.1 - Oscillatory characteristics of the AG.

Figure 10.1 shows three oscillatory characteristics corresponding to different bias voltages. Characteristic 1 corresponds to the displacement at which the slope of the current-voltage characteristic has the greatest value. As the voltage rises U m in the average slope drops and the slope decreases.

Characteristic 2 corresponds to a lower bias voltage, at which the static slope of the I – V characteristic of the amplifying element at the operating point is less than the maximum slope. As a consequence, with increasing voltage, the average slope S Wed grows and only at very large values U m in begins to decrease.

The third characteristic corresponds to the case when, in the absence of an input signal, no current flows through the amplifying element. This current, and hence the current in the oscillatory circuit, appears only at a certain voltage amplitude U m in sufficient to turn on the lamp or transistor during part of the high frequency oscillation period.

Feedback lines

These lines define the dependence of the amplitude U m in, i.e., the output voltage of the feedback circuit, from the amplitude of the current I m 1, which is the input current of this circuit:.

Insofar as and we get

.

It follows that the feedback lines are graphically depicted as straight lines starting from the origin (Figure 10.2). The slope of these straight lines is different and depends on the value of the coefficient To wasps... The stronger the feedback in the oscillator, the smaller the angle of inclination of the feedback line relative to the axis U m in(in figure 10.2 ).

Figure 10.2 - Feedback lines.

10.3 Determination of stationary vibration amplitude

In stationary mode AG, the amplitude of the input voltage U m in and the amplitude of the first harmonic of the output current corresponding to this mode I m 1 of the amplifying element must simultaneously satisfy both of the specified dependencies. This is possible only at the points of intersection of the oscillatory characteristic and the feedback line. In fig. 10.3 abscissa axis of vibration characteristic U m in serves simultaneously as the ordinate axis of feedback lines 2-5, and the scale on them is the same. The common axis of ordinates of characteristic 1 and lines 2-5 is the current I m 1.

The feedback line 2, corresponding to the gain of the feedback loop, has a common point with the oscillatory characteristic 1 only at the origin. In this case, self-excitation of the autogenerator does not occur due to the small coefficient To wasps or a small value of the resonant resistance of the circuit R res.

Figure 10.3 - Determination of the stationary state of the AG in the mode of soft self-excitation.

At a critical coefficient, the forward feedback 3 merges with the oscillatory characteristic in the OA region, in which it is linear, but does not intersect this characteristic.In this case, self-excitation is also absent, which confirms the conclusion: in an oscillator operating in a linear mode and having, it is impossible to obtain self-oscillations ...

Oscillations in the AG arise only with a coefficient corresponding to the feedback line 4. Under the conditions of a soft self-excitation mode, this line has two common points with an oscillatory characteristic, 0 and B. Point B corresponds to the stationary state of the oscillator, characterized by current amplitudes I m 1 B and voltage U m in... The generator comes to this state in the process of self-excitation, but can leave it under the influence of various destabilizing factors.

Consider the processes that will take place at the same time.

Suppose that the voltage at the input of the amplifying element has decreased to the value U m inxC... This voltage will cause a current in the generator output circuit I m 1 C(point C in Figure 10.3), which, thanks to the feedback, will increase the voltage at the input to U m in, which will lead, according to characteristic 1, to an increase in current up to I m 1 A and so on. As a result, the generator will return to the state defined by the point B of intersection of characteristics 1 and 4. Similarly, it can be shown that if, for any reason, the voltage at the input of the amplifying element increases and becomes greater than U m in(point D in Figure 10.3), the generator will automatically return to the state defined by point B. The above reasoning confirms that point B is a point of stable equilibrium and corresponds to the stationary mode of operation of the generator. The amplitudes of voltage and current in the stationary mode are determined by the magnitude of the feedback. With increasing feedback (Figure 3, line 5), the corresponding stationary amplitudes increase to values U m in and I m 1 E.

The second common point of the oscillatory characteristic 1 and the feedback line 4 (Figure 10.3, point 0) is unstable, since the oscillations that have arisen in it, regardless of the initial amplitude, increase to oscillations with stationary amplitudes determined by the position of point B.

Figure 10.4 - Determination of the stationary state of the AG in the hard self-excitation mode.

Under the conditions of a severe self-excitation mode (Figure 10.4), the oscillatory characteristic 1 and the feedback line have three common points: O, A, B. Point 0 characterizes the steady state of rest of the auto-generator, i.e., the absence of self-excitation at small initial amplitudes of oscillations. Oscillation occurs only when the initial amplitude of the input voltage becomes larger U m in defined by point A in Fig. 10.4, for example, the voltage increased to a value U m inxC... The current caused by this voltage I m 1 C will use feedback to increase the voltage at the generator input, which will lead to a greater increase in current, etc.

(see figure 10.4, lines with arrows). As a result, a stable oscillatory mode (point B) is achieved, characterized by the amplitudes U m in and I m 1 B.

Suppose now that the voltage at the generator input has become less than U m in and reached the value U m in defined by point D. Then the current will decrease to I m 1 D, which will cause a further decrease in the input voltage, as shown by the lines with arrows in Fig. 4. As a result, the oscillations are damped. Consequently, the point A of intersection of the oscillatory characteristic and the feedback line characterizes the unstable state of the oscillator mode.

Study questions:

1Amplitude characteristics of self-excitation modes

4 Discontinuous generation

1 Amplitude characteristics of self-excitation modes

In order to trace in more detail the process of the appearance, growth and establishment of oscillations in the oscillator, it is convenient to use the graphical method using the oscillatory characteristic and the feedback line.

Oscillatory characteristic the dependence of the amplitude of the first harmonic of the collector current on the amplitude of the control voltage based on the transistor Ik1 = f (UBE) is called. The type of the oscillatory characteristic depends on the position of the operating point on the transistor transmission characteristic Ik = f (ebe).

When the transistor operates in the mode of oscillation of the first kind, that is, when the operating point A is selected in the middle of the linear section of the transmission characteristic, as shown in Fig. 2.10, a, the vibrational characteristic has a convex shape (Fig. 2.10,6,1). With an increase in the amplitude of the input voltage, the amplitude of the output current first increases rather rapidly due to the constancy of the slope Sd = const). Then the rise in the output current slows down due to the non-linearity of the lower and upper bend of the transistor characteristic.

If the operating point on the transient characteristic of the transistor is selected in the cutoff region of the output current B (oscillation mode of the second kind), then the oscillatory characteristic starts slightly to the right of zero. Then, as the input (control) voltage increases, the vibrational characteristic has a lower bend corresponding to the nonlinear lower section of the flow characteristic and, accordingly, an upper bend (Fig. 2.10,6,11).

Feedback line the graphically expressed dependence of the feedback voltage on the current in the output circuit of the transistor is called. Since the feedback loop is linear, the feedback line is a straight line rising from the origin (Figure 2.10, c).

To trace the process of occurrence, growth and establishment of oscillations, we combine the oscillatory characteristic and the feedback line on the same graph.

2 Soft self-excitation mode.

Soft self-excitation mode... In fig. 2.11, and the amplitude oscillatory characteristic of the generators in the oscillation mode of the first kind (curved line) and the amplitude characteristic of the feedback of the oscillator (straight line) are combined in one graph. Since the initial operating point is located on the middle steep section of the transistor's throughput characteristic (see Fig. 2.10, a), even the smallest voltage changes at the input of the transistor will cause changes in the output current. And such small voltage changes in the circuit are always either due to fluctuations of charge carriers, or due to switching on the voltage of the power source.

Let us assume that a current Ib1m appeared in the circuit due to fluctuations (Fig. 2.1 \, a). This feedback current creates an excitation voltage U1 at the input. This voltage, in accordance with the oscillatory characteristic, causes a current I2 in the output circuit. At current I2, voltage U2 is induced on the input circuit of the oscillator in accordance with the feedback line, which causes current I3, and so on. The sequence of increasing oscillations is shown in Fig. 2.11, and arrows. So, the oscillations in the circuit will increase to the value determined by the point B of the intersection of the oscillatory characteristic and the feedback line. Point B corresponds to the steady-state oscillation mode: a current Iset flows in the output circuit, a voltage Uset is created in the base-emitter section. At point B, the amplitude balance is performed, and stable oscillations are established in the oscillator.

Indeed, if at (the output of the auto-generator the current has decreased to the value of I3, then through the feedback circuit it will create a voltage U3 at the input and the oscillations will again increase to a steady value. If due to an external influence the current in the circuit increases, for example, to the value Iv then the losses in the loop turn out to be higher and the voltage to the input through the feedback loop is less induced.

From what has been considered, it follows that in the section where the oscillatory characteristic passes over the communication line, there are more losses and fluctuations increase. In the area where the oscillatory characteristic is below the feedback line, the replenishment is less than the loss and the oscillation is reduced. At point B, the intersections of the amplitude characteristics of the replenishment are equal to the losses.

Thus, in the mode of oscillations of the first kind, oscillations in the auto-generator arise after switching on the power source independently and grow to a steady-state value smoothly, softly. Therefore, this mode of vibration is called the soft mode of self-excitation.

3 Hard mode of self-excitation.

Hard self-excitation mode. If the operating point on the transistor throughput characteristic is selected in the output current cutoff region, the oscillatory characteristic intersects with the feedback line at two points, as shown in Fig. 2.11, b.

In region 1, the curve passes under a straight line - this means, as shown above, that the losses in the circuit exceed the energy replenishment and oscillations do not arise. In area 2, the curve goes over the straight line - this means that the losses in the loop are less than the replenishment, and the fluctuations can increase. It can be seen from this that in the mode of oscillations of the second kind, oscillations automatically, from fluctuations, cannot arise (section 0-1 in Fig. 2.11, b). For the occurrence of oscillations in the oscillator in the oscillation mode of the second kind, it is necessary to apply a voltage of significant amplitude UB03b> Un to the input circuit of the transistor. Only after this sharp, hard external voltage jump, oscillations arise and grow rapidly. Hence, the self-excitation mode is called hard. Oscillations increase to a steady-state value corresponding to point B of stable oscillations.

To reveal the features of self-excitation of the generator and to determine the stationary amplitude of the output oscillations, it is convenient to use the method of joint analysis of the amplitude characteristic of the amplifier K and the straight line of the OS β = U OS / U OUT, reflecting the effect of the PIC circuit (Fig. 5). Note that the amplitude characteristic of the amplifier itself in the theory of generators corresponds to the oscillatory characteristic. The essence of the method is traditional and lies in the fact that the generator circuit (see Fig. 3) is mentally (and essentially) divided into two circuits - linear and nonlinear. The linear circuit represents the PIC loop, and the nonlinear circuit represents the amplifier itself (op amp and OOS circuit).

Soft self-excitation mode... Typical form of the amplitude characteristic of a nonlinear amplifier based on an op amp (Fig. 5, a). With a small amplitude of the input voltage U OUT / U IN = K. With an increase in the amplitude, the nonlinearity of the transfer characteristic of the op-amp begins to appear, and the gain K (and hence the output voltage) will be practically constant and may even decrease. On the linear section, the OS voltage U OS = U BX is linearly related to the output voltage U OUT and is determined by the transmission coefficient of the POS circuit β (after all, U OS = β U OUT). This voltage acts at the input of the amplifier, therefore, the OS line (dependence of U OUT from U OS) is drawn on the graph in the form of a straight line β at an angle γ = arctan (l / β) to the abscissa axis (see Fig. 5, a).

Suppose that a small input voltage U BX1 acts on the input of the amplifier. Then, after amplification by K times, the voltage U OUT1 will appear at the generator output. This voltage, weakened by the positive feedback circuit by a factor of β, is fed to the input of the amplifier in the form of a voltage U BX2. It will then, in turn, increase to the voltage U OUT2. A similar process will continue until the amplitude of the output oscillation reaches a stationary value, at which the amplitude balance condition is satisfied.

Stationary the amplitude of self-oscillations of the generator can be determined by the coordinates of the point of intersection of the amplitude characteristic of the amplifier with the line of the feedback (point A in Fig. 5, a). Point A is a point of stable equilibrium, and with a random deviation of the output voltage amplitude from the stationary value U CT, the autogenerator always returns to its original state. Let us assume that the amplitude of the output voltage U OUT has decreased relative to U CT by the value ∆U OUT. This will cause a decrease in the OS voltage U OS by the value ∆U OS, which, in accordance with the amplitude characteristic, in turn, will lead to an increase in the output voltage U OUT. In this case, the output voltage will grow to a stationary value U ST, and the OS voltage instability ∆U OS will decrease to zero and go to the point U OSST. Let us investigate the influence of the value of the transfer coefficient of the POS circuit β on the self-excitation mode of the autogenerator of harmonic oscillations with the type of the amplitude characteristic of the amplifier shown in Fig. 5 B. By the way, we note that the change in the value of the transmission coefficient of the POS circuit β in the circuit in Fig. 3 can be carried out either by adjusting the value of the resistance of the resistor R, or by changing the switching coefficient of the oscillatory circuit (incomplete switching on the circuit).

If we smoothly increase the transmission coefficient β (decrease the slope of the β line), then, starting from a certain critical value βcr, the amplitude of the stationary oscillation f / CT will increase (see Fig. 5). This mode of self-excitation of the generator is called soft. To ensure it, the amplitude characteristic of the amplifier must go out of zero and have a sufficiently large angle of inclination to the abscissa axis at the origin. The soft mode is characterized by the fact that by selecting the transfer coefficient β it is possible to set any, very small (close to the noise level), stationary amplitude of the output oscillations. In the soft self-excitation mode, oscillations occur at the generator output when low levels of noise voltages appear at the amplifier input.

Fig. 5. Oscillator soft self-excitation mode:

a - amplitude characteristic and feedback line;

b - dependence of the amplitude U on the transmission coefficient β

Hard self-excitation mode. Different picture of processes

observed in processes in autogenerators, the amplitude characteristic of the amplifier is S-shaped (Fig. 6, a). The amplifier has such an amplitude characteristic when its operating point is located in the nonlinear section of the op-amp transfer characteristic. For self-excitation of autogenerators, a very strong PIC is required, and the output oscillations arise instantly - in a jump. A sharp ("explosive") self-excitation of the oscillator occurs at a value of the transfer coefficient of the feedback circuit β = β 1 when the feedback line (line 1 in Fig. 6, a) touches the bottom of the amplitude characteristic at point 0. The generation of oscillations breaks down abruptly at the value of the transfer coefficient β, less than β 2, when the line OS (line 2) becomes tangent to the convex part of the amplitude characteristic. In the graphs in Fig. 6, a point A reflects the stationary mode of operation of the autogenerator, and point C - the mode of unstable equilibrium. This situation is explained as follows: at the amplitudes of the output oscillations of the oscillator located on the graphs below point C, the oscillations damp, and at amplitudes above point C, they will increase and reach a stationary amplitude at point A.