The self-excitation mode, in which, after turning on the power source, the oscillations gradually increase, is called soft self-excitation; if any additional influence is required to excite the oscillations, then this mode is called hard.
Rice. 13.2. Change in slope during soft self-excitation mode
The implementation of a soft self-excitation mode can be achieved by appropriately selecting the bias voltage in the section of the current-voltage characteristic of the transistor with a high transconductance.
This mode corresponds to the dependence S=f(U mb) of the following form, shown in Fig. 13.2.
In the same fig. a direct line was drawn 
. For the point of intersection of the graphs, the amplitude balance equation is satisfied and the steady-state amplitude of the oscillation is equal to 
. In the soft mode, the stationary mode is stable, while the rest mode is unstable. Therefore, self-excitation of the self-oscillator occurs.
A characteristic feature of the hard mode is that small fluctuations at the transistor input cannot cause self-excitation of the self-oscillator; self-excitation is possible only with a large initial voltage amplitude. This mode is implemented by applying a blocking bias voltage to the UE, at which small amplitudes of the input voltage cannot cause a current in the output circuit of the UE.
This mode is characterized by the following dependence S=f(U mb), shown in Fig. 13.3.
Rice. 13.3. Change in slope during hard self-excitation mode
Mode corresponding to the oscillation amplitude 
, is stable, and the mode corresponding to the amplitude 
, unstable.
13.3. Equivalent three-point oscillator circuits
The simplest self-generators in configuration are self-generators operating according to a three-point circuit. In such self-oscillators, the transistor with its three terminals is connected to three points of an oscillatory circuit consisting of three reactive elements.
A generalized three-point oscillator circuit is shown in Fig. 13.4.
Rice. 13.4. Generalized equivalent circuit of a self-oscillator
For self-oscillations to occur, it is necessary that:
Depending on which reactive elements quantitatively predominate in the circuit, self-oscillators are distinguished, built according to the inductive (Fig. 13.5) and capacitive (Fig. 13.6) three-point circuits.
Inductive three-point:
Rice. 13.5. Inductive three-point
,
,
.
Capacitive three-point:
Rice. 13.6. Capacitive three-point
- frequency of generated oscillations.
, 
,
.
Feedback coefficient through three-point circuit elements:
.
For inductive three-point: 
.
For capacitive three-point: 
.
Clapp scheme
In a modified three-point capacitive circuit, higher frequency stability is achieved (Fig. 13.7).
Rice. 13.7. Clapp scheme
The introduction of capacitor C 3 reduces the inclusion factor of the transistor in the circuit, reducing the destabilizing effect of its parameters on the frequency of the self-oscillator.
, Where 
.
In all circuits, the circuit is partially connected to the collector circuit of the transistor.
The coefficient of inclusion of the circuit in the collector circuit:
Equivalent collector circuit resistance: 
.
Study questions:
1Amplitude characteristics of self-excitation modes
4 Intermittent generation
1 Amplitude characteristics of self-excitation modes
In order to trace in more detail the process of occurrence, growth and establishment of oscillations in a self-oscillator, it is convenient to use the graphical method using the oscillatory characteristic and feedback line.
Oscillatory characteristics is called the dependence of the amplitude of the first harmonic of the collector current on the amplitude of the control voltage at the base of the transistor Ik1 = f(UBE). The type of oscillatory characteristic depends on the position of the operating point on the pass characteristic of the transistor Ik=f(ebe).
When the transistor operates in the oscillation mode of the first kind, i.e. when operating point A is selected in the middle of the linear section of the flow characteristic, as shown in Fig. 2.10a, the oscillatory characteristic has a convex shape (Fig. 2.10,6,1). As the amplitude of the input voltage increases, the amplitude of the output current initially increases quite quickly due to the constant slope Sd = const). Then the growth of the output current slows down due to the nonlinearity of the lower and upper bend of the transistor characteristic.
If the operating point on the transition characteristic of the transistor is selected in the cutoff region of the output current B (second-order oscillation mode), then the oscillatory characteristic begins slightly to the right of zero. Then, as the input (control) voltage increases, the oscillatory characteristic has a lower bend, corresponding to the nonlinear lower section of the throughput characteristic, and, accordingly, an upper bend (Fig. 2.10,6,11).
Feedback line is called the graphically expressed dependence of the feedback voltage on the current in the output circuit of the transistor. Since the feedback circuit is linear, the feedback line is a straight line ascending from the origin (Fig. 2.10c).
To trace the process of occurrence, growth and establishment of oscillations, we combine the oscillatory characteristic and the feedback line on one graph.
2 Soft self-excitation mode.
Soft self-excitation mode. In Fig. 2.11, and the amplitude oscillatory characteristic of the generators in the first-order oscillation mode (curved line) and the amplitude feedback characteristic of the self-oscillator (straight line) are combined on one graph. Since the initial operating point is located on the middle steep section of the transistor's pass-through characteristic (see Fig. 2.10, a), even the smallest changes in voltage at the transistor input will cause changes in the output current. And such small changes in voltage in the circuit are always present either due to fluctuations of charge carriers, or due to the inclusion of the voltage of the power source.
Let us assume that a current Ib1m appears in the circuit due to fluctuations (Fig. 2.1\a). This current through the feedback circuit 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 self-oscillator in accordance with the feedback line, which causes current I3, etc. The sequence of increase in oscillations is shown in Fig. 2.11, and by arrows. Thus, oscillations in the circuit will increase to a value determined by point B of the intersection of the oscillatory characteristic and the feedback line. Point B corresponds to the steady-state oscillation mode: current Iset flows in the output circuit, and voltage Uset is created in the base-emitter section. At point B, the amplitudes are balanced, and stable oscillations are established in the self-oscillator.
Indeed, if at the output of the self-oscillator the current has decreased to the value I3, then through the feedback circuit it will create voltage U3 at the input and the oscillations will again increase to the steady-state value. If, due to external influence, the current in the circuit increases, for example, to the value Iv, then the losses in the circuit are greater and the voltage induced at the input through the feedback circuit is less.The oscillations decrease to a steady-state value.
From what has been considered it follows that in the section where the oscillatory characteristic passes over the communication line, the replenishment is greater than the losses and the oscillations increase. In the area where the oscillatory characteristic is below the feedback line, the replenishment is less than the loss and the oscillations are 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 self-oscillator arise independently after turning on the power source and increase smoothly and softly to a steady value. Therefore, this oscillation mode is called the soft self-excitation mode.
3 Hard self-excitation mode.
Severe self-excitation mode. If the operating point on the pass characteristic of the transistor 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 the straight line - this means, as shown above, that losses in the circuit exceed energy replenishment and oscillations do not occur. In region 2, the curve passes above the straight line - this means that losses in the circuit are less than replenishments, and fluctuations can increase. From this it is clear that in the mode of oscillations of the second kind, oscillations cannot arise automatically from fluctuations (section 0-1 in Fig. 2.11, b). For oscillations to occur in a self-oscillator in the mode of oscillations 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, severe external voltage surge, oscillations arise and quickly increase. Hence the self-excitation mode is called hard. The oscillations increase to a steady value corresponding to point B of stable oscillations.
The soft mode is characterized by the unconditional rapid establishment of a stationary mode when the autogenerator is turned on.
The hard mode requires additional conditions to establish oscillations: either a large feedback coefficient or additional external influence (pumping).
In an AG with a soft mode, the position of the operating point does not depend on the developing oscillations. For the best excitation, it is desirable that the operating point of the active element is in the middle of the linear section of the DPC, that is, at the point of maximum gain (Fig. 10).
In an AG with a hard excitation mode, the operating point is set in the region of the lower nonlinear section (close to the cutoff) so that the current in the absence of generation would be close to zero. Due to the low gain, initial oscillations may not develop (Fig. 11).
To analyze the quality of the excitation mode, the so-called oscillatory characteristics of the AG are used: the dependence of the amplitude of the output voltage of the amplifier (or gain) on the amplitude of the input voltage when the AG circuit is open, and the opening can be done at any convenient point, for example, as shown in Fig. 12.
The soft self-excitation mode is characterized by a constantly concave curve with a maximum steepness at the origin. The feedback circuit in the coordinates of the oscillatory characteristic is called the feedback line (FLO). Since in the coordinates the equation of the feedback line has the form , and
in coordinates, the VOC is a line parallel to the abscis axis.
The point of intersection of the nonlinear oscillatory characteristic with the VOC, in accordance with the amplitude balance equation, determines the stationary amplitudes and.
In Fig. Figure 13 shows a typical view of the oscillatory characteristics of an AG with soft self-excitation and several VOCs.>
From Fig. 13 it is clear that when the oscillatory characteristic and the VOC have two intersection points ABOUT And M, and t. ABOUT is unstable and M– stable.
Indeed, let us consider the case in which stable generation occurs (Fig. 14).
Suppose that when the AG is turned on, at some point in time, a voltage with amplitude appears at the output. This oscillation is transmitted through a feedback circuit to the input with amplitude . In turn, the voltage will cause a voltage at the output (see arrows in Fig. 14), etc. Then we can make a transition from the oscillatory characteristic to the VOC, from the VOC to the oscillatory characteristic, etc., until we finally reach the point M. These kinds of graphs are called Lameray diagrams. This diagram shows that any, no matter how small, disturbance when the AG is turned on leads it to a stationary state, determined by point M. In Fig. 15 shows that t. M is stationary when the amplitude changes from .
Similar diagrams can be viewed using Fig. 13, b.
To change the value of the stationary amplitude in an AG with a soft mode, it is enough to change the value of the feedback coefficient. When increasing To os from zero, self-oscillations do not occur until the value To os will not reach the value To os,cr =1/ K(0), Where K(0) is the gain at , i.e., when the AG is excited. Further increase To os leads to an increase (see Fig. 16). Decrease To os leads to a change along the same line as when increasing. You can construct a similar graph for the signal amplitude in the output circuit of the AG.
The hard mode of self-excitation of the AG is characterized by a concave-convex oscillatory characteristic with one or several inflection points and, accordingly, more than two intersection points (Fig. 17).
This type of characteristic is typical for AGs whose operating point of the amplifying element is located at the lower bend of the flow characteristic. It is easy to show that points O and M are stable in this case, and point N is unstable. As long as the amplitude at the output is less, self-oscillations will not increase (see diagram, Fig. 18)
In order to transfer the generator to state M at a given K os, it is necessary to energize the AG with an additional signal (from the input or output side), called the excitation or pump signal. In this case, the magnitude of the pump signal must exceed the value determined by point N. In this case, the pump signal through the feedback circuit will lead the generator to a stationary state M (see Fig. 19).
To excite such an AG, it is possible not to use additional pumping, but to establish such strong feedback that the generator is self-excited; in this case the condition must be met. Considering that in this case K(0) is quite small, the condition can only be met with very deep feedback. This fact is illustrated in Fig. 20.
While , the amplitude of self-oscillations is zero. When in a generator
oscillations with amplitude will be established. Further increase To os will lead to a smooth decrease in amplitude. If we now reduce To os, then the amplitude of oscillations from the input side of the amplifier will gradually decrease until the feedback coefficient reaches the value at which the VOC touches the convex part of the oscillatory characteristic. The amplitude of stationary oscillations will be equal to . Further decrease To os will lead to disruption of self-oscillations. Thus, in an AG with a rigid oscillatory characteristic it is impossible to establish oscillations with an amplitude less than . Similar reasoning can be made for the amplitude of the AG output signal, but since there is an unambiguous connection between the input and output amplitudes, this need not be done.
The advantage of the soft self-excitation mode is the ease of bringing the AG into the required stationary mode. The disadvantage is low efficiency due to the large value of the direct current component. In an AG with a hard mode, the advantage is the absence of direct current (or its small value) in the AG rest mode.
Using automatic bias circuits in the input circuit, it is possible to achieve a combination of the advantages of both types of excitation: at the moment of startup, the operating point is at the point of maximum slope (in the middle of the linear section), and with increasing amplitude, the operating point shifts towards the cutoff due to the rectifying properties of the input p-n -transition and automatic displacement chain. An example of a schematic diagram of such an AG is shown in Fig. 21.
A constant voltage is released at resistance Rb, proportional to the amplitude of the oscillation supplied to the input. In Fig. Figure 22 shows a picture of the transition of AG to a stationary state.
The steady-state mode is characterized by the operation of the transistor with a cutoff angle of 90 0. Thanks to the oscillatory circuit of the amplifier, harmonic self-oscillations develop at the output.
Depending on the values of the constant supply voltages supplied to the electrodes of the amplifying element and on the coefficient K os, two self-excitation modes are possible: soft and hard.
1.Soft self-excitation mode.
In this mode, operating point A is selected on the linear section of the current-voltage characteristic of the amplifying element, which ensures the initial operating mode of the amplifying element without cutting off the output current i out (Fig. No. 2).
Rice. No. 2. Diagram of soft self-excitation mode.
Under these conditions, self-excitation arises from the most insignificant changes in the input voltage Uin, which are always present in real conditions due to fluctuations of charge carriers.
At first, oscillations in the autogenerator increase relatively quickly. Then, due to the nonlinearity of the current-voltage characteristic of the amplifying element, the growth of the oscillation amplitude slows down, since the voltage at its input falls on sections of the current-voltage characteristic with an increasingly lower static slope, and this leads to a decrease in the average slope S avg and the transmission coefficient K os of the reverse circuit communications.
An increase in oscillations occurs until the transmission coefficient K decreases to unity. As a result, a stationary mode is established in the self-oscillator, which corresponds to a certain amplitude of output oscillations, and the cutoff angle of the output current is 0>90 0 . The frequency of these oscillations is very close to the resonant frequency of the oscillatory system.
If the amplifying element had a linear current-voltage characteristic, the amplitude of self-oscillations would increase 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 i out of the amplifying element is non-sinusoidal. However, with a sufficiently high quality factor (50...200) of the oscillatory system, the first harmonic of this current and, consequently, the voltage at the output of the self-generator are almost harmonic oscillations.
2. Hard self-excitation mode.
In this mode, the bias voltage U 0 is set so that at small amplitudes of the input voltage, the current does not pass through the amplifying element. Then the slight oscillations that arise in the circuit cannot cause a current in the output circuit, and self-excitation of the self-oscillator does not occur. Oscillations occur only when their initial amplitude is sufficiently large, which cannot always be ensured. The process of occurrence and growth of oscillations in a hard self-excitation mode is illustrated using Fig. No. 3.
Fig. No. 3. Hard self-excitation diagram
From examination of this figure it is clear that at small initial amplitudes of the input voltage (curve 1), the current i out = 0 and self-oscillations do not occur. They arise only at a sufficiently large initial voltage amplitude (curve 2) and quickly increase to a steady-state value. In stationary mode, the amplifying element operates at an output current cutoff angle of 0<90 0 .
For ease of operation of the autogenerator, it is more advisable to use a soft self-excitation mode, since in this mode oscillations occur immediately after turning on the power source. However, in a rigid oscillation mode with a cutoff angle of 0<90 0 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 тока усилительного элемента.
Automatic offset. Its use makes it possible for the self-oscillator to operate in the soft self-excitation mode upon initial switching on, followed by an automatic transition to the hard self-excitation mode. This is achieved by using a special automatic bias circuit in the autogenerator.
Fig. No. 4a shows a simplified circuit diagram of a self-oscillator based on a bipolar transistor VT, the load of which is the oscillatory circuit L2C2. A positive feedback voltage is created across coil L1 and is applied between the base and emitter of the transistor. The initial bias voltage6 at the base of the transistor is created by the source turned on by the auto-bias circuit R1C1.
The process of occurrence and growth of oscillations is illustrated using Fig. No. 4b. At the first moment after turning on the generator, i.e. at the moment of the appearance of oscillations, operating point A is located in the area of the maximum steepness of the current-voltage characteristic of the transistor. Thanks to this, oscillations occur easily under conditions of a soft self-excitation mode. As the amplitude increases, the base current increases, the constant component of which creates a voltage drop U cm across resistor R1 (the alternating component of this current passes through capacitor C1). Since the voltage U cm is applied between the base and emitter in negative polarity, the resulting constant voltage at the base U 0 - U cm decreases, which causes the operating point to shift downward along the transistor characteristic and switches the self-oscillator to operating mode with small cutoff angles of the collector current while the currents collector i k and base i b have the form of a sequence of pulses, and the voltage at the output U out, created by the first harmonic of the collector current, is a sinusoidal oscillation with a constant amplitude.
Thus, the automatic bias circuit R1C1 in the self-oscillator acts as a regulator of the self-excitation process and initially provides conditions for soft self-excitation with a subsequent transition to a more favorable mode with small cut-off angles.
If in a self-oscillator 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 gradually increase.
This mode of self-excitation is called light.
To obtain an easy mode, it is necessary that the oscillatory characteristic leaves the zero point and has a sufficiently large slope in the region of small amplitudes. All these requirements are met when using automatic offset. When using forced (external) bias, the oscillatory characteristic takes the form:
For oscillations to occur in this case, very strong feedback is required (line OA, mutual induction M 1).
After the oscillations have been established, the connection can be weakened to the value M2, at which the communication line occupies the OB position. With further weakening of the connection, the oscillations break down. To restore oscillations M corresponding to the communication line OA. This mode of self-excitation is called hard.
Purpose, classification and principles of constructing synchronization systems.
In most cases, the normal functioning of various information transmission systems requires ensuring a certain synchronization of the operation of transmitting and receiving equipment. This function is usually assigned to special synchronization systems. Their noise immunity and the quality of operation of the transmission system as a whole depend on their noise immunity and the quality of their work. Synchronization systems generate special synchronizing signals on the receiving side, synchronous with the corresponding signals generated on the transmitting side, taking into account the distortions that appear during the propagation of signals along the transmission channel.
The whole 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 channels with variable parameters.
Optimal reception of signals with random parameters requires estimation (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 synchronization tasks in transmission systems can be provided by three synchronization systems: high-frequency, element-by-element (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 up to phase.
The task of element-by-element (clock) synchronization is to ensure on the receiving side the fixation of the time boundaries of elemental signals 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 post-detector signal processing and signal separation into their channels.
In analog transmission systems, such elementary signals are usually channel intervals (time intervals allocated for transmission over one channel), and in digital systems - elementary information symbols.
Group synchronization must ensure the fixation of the time boundaries of certain groups, elementary signals, for example words, cycles, frames, etc.
In some systems, all three of these types of subsystems can operate simultaneously.
High-frequency and element-by-element synchronization signals usually have a periodic structure. Group synchronization signals can be either periodic or form a random stream. In digital transmission systems with cyclic and periodic polling, when all three of these types of synchronization can operate, the frequencies of all of the listed types of synchronization can be selected as multiples of each other.
For example, each frame 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 carried out after frame synchronization has been set.
