Study of the operation of the simplest tube generator of electromagnetic oscillations. Stable band generator
In 1913, A. Meissner invented a remarkable method for generating continuous electrical oscillations using an electron tube (§ 53). Electronic circuit tube generator vibrations is shown in Fig. 405. Oscillatory circuit connected to the anode and cathode of a three-electrode lamp. Next to the oscillating circuit coil, a second coil is wound on the same frame, one end of which is also connected to the cathode of the lamp, and the other end is connected to the lamp grid. At making the right choice lamp mode, this setting, after the initial “push” imparted by closing the circuit, produces undamped electrical oscillations with a frequency determined by the capacitance and self-inductance of the circuit.
Rice. 405. Scheme of using a triode for self-excitation of continuous electrical oscillations.
Self-excitation of oscillations is produced by an electron tube as follows. At the initial moment, following the closure of the anode circuit, the electron flow rushes inside the lamp from the cathode to the anode and in the external circuit from the anode through the coil of circuit 1 to the cathode. Rapidly increasing, the current creates, passing through the circuit coil, a magnetic field, which, at the moment of its formation, induces grid coil 2 electromotive force in such a direction that the lamp grid acquires a positive potential in relation to the cathode. The appearance of a positive potential on the grid instantly increases the current passing through the lamp and through the coil
contour. This entails a new sharp (even faster than at the first moment after closing the circuit) increase magnetic field. In the grid coil, an electromotive force is again induced in the same direction as before, but even greater in magnitude, proportional to the greater rate of increase of the magnetic field; the positive potential of the grid increases. An increase in the positive potential of the grid instantly affects the increase in the anode current, etc. Thus, in the first stage of the process considered, the increase in current positively charges the grid, which in turn increases the current.
But this first stage of the process soon leads to a “crisis” and ends. It breaks off when, at some stage of current increase, the rate of current increase is less than that at the previous stage. The magnetic field of the loop coil, increasing at a lower speed than before, gives in the grid coil an electromotive force of the same direction as before, but of a smaller magnitude. The grid potential, remaining positive, will decrease, which will cause a decrease in the current and stop the growth of the magnetic field of the loop coil. The electromotive force in the grid coil is no longer induced, and the grid potential instantly drops to zero. As a result, the current decreases sharply, the magnetic field of the loop coil quickly decreases and induces an electromotive force in the grid coil, directed oppositely to the previous one. The grid acquires a large negative potential and immediately “locks” the lamp - it stops the current through it, turning it into a non-conductor. Thus, in the second stage (shorter than the first), a crisis drop in the potential of the grid occurs, ending with the grid receiving a large negative potential and locking the lamp.
Now the loop capacitor comes into play. The lamp is locked, and the loop coil has stored magnetic energy. The magnetic field of the coil, disappearing, creates an extra current that charges the capacitor; the flow of electrons, whose path through the lamp is blocked, is concentrated on the capacitor plates connected to the cathode.
The plates connected to the anode acquire a high positive potential. This completes the third stage.
At a subsequent point in time, the capacitor is discharged. Through the loop coil, the electron flow flows back to the anode; although the magnetic field of the coil increases again, its polarity is opposite to the previous one, and therefore the electromotive force induced in the grid coil has such a direction that the grid potential remains negative; the lamp remains locked. By the time the potentials at the capacitor terminals become equal, the magnetic field of the coil will reach its maximum (end of the fourth stage).
From this moment, due to the transition from an increase in the magnetic field to its decrease, the direction of the electromotive force induced in the grid coil changes. The grid, as in the first stage, acquires a positive potential and opens the lamp, but the lamp remains inactive for some time, since the electromotive force of the self-induction of the loop coil compensates for the electromotive force of the battery; the voltage at the anode is low and the anode current is correspondingly low. The magnetic field of the loop coil, disappearing, drives electrons to the capacitor plates connected to the anode; a stream of electrons coming from the lamp, which begins to operate, soon rushes there. Instantly, a high negative potential arises here (end of the fifth stage).
In the subsequent, sixth, stage of the process, the phenomena that occurred in the first stage are repeated with increased intensity: the discharge current of the capacitor and the current passing through the lamp simultaneously flow in the loop coil.
Rice. 406. Three-point circuit of a tube oscillator
The stronger the electrical oscillations in the lamp generator “self-oscillate,” the more tightly the lamp is locked at the right moment by the high negative potential of the grid. Energy dissipation during oscillations is automatically replenished by the energy of the anode battery. The amplitude of oscillations is limited by the lamp power; To increase power, several lamps are connected in parallel.
Generator vacuum tubes designed for power have a saturation current exceeding 5-10 A at anode voltage
In the classical Meissner circuit we considered, the voltages applied to the lamp grid are taken (in in this case through inductive coupling of coils 1 and 2) from the anode circuit. This principle of excitation of voltages in the grid circuit by borrowing them from the anode circuit is called the principle feedback. Various modifications of the scheme are possible. Instead of inductive feedback, capacitive feedback can be used. The so-called three-point scheme is often used, in which part of the contour coil serves as a mesh coil (Fig. 406).
Mathematical analysis of self-excitation of oscillations shows that the mutual inductance of the coils providing feedback must be no less than the value determined by the inequality
Where active resistance, capacitance and inductance of the oscillatory circuit of the anode circuit, gain and slope of the grid characteristic of the lamp.
Thus, self-excitation of oscillations occurs at a lower value of mutual feedback inductance, the greater the gain and transconductance of the lamp and the smaller all the parameters of the oscillatory circuit: its active resistance, capacitance and inductance.
Tube generators are often powered by dynamos, which provide current to heat the lamps and high voltage to power the anode circuits. Conventional alternating current is often used: the filaments of heating lamps can be heated directly with alternating current, and the same high voltage To power the anode circuits, it is produced using a transformer and a lamp rectifier (kenotron).
Since the frequency of oscillations generated in the circuit is somewhat influenced by the operating mode of the lamp, in order to avoid accidental changes in frequency associated with changes in the operating mode of the lamp, so-called piezoquartz frequency stabilizers are used.
A small plate, suitably cut from a quartz crystal (§ 23), is placed in a capacitor K connected to the lamp grid (Fig. 407). Electrical vibrations cause forced mechanical vibrations of the piezoquartz plate. When the frequency of potential oscillations applied to the plate is close to the natural frequency of the mechanical vibrations of the plate, resonant swinging of the plate oscillations occurs. Oscillatory changes in the thickness of the piezoquartz plate are, in turn, accompanied by the appearance of charges on its faces, changes in the magnitude and sign of which support potential oscillations on the plates of the grid capacitor K. Thus, random changes in the frequency of electrical oscillations supplied to the capacitor K have almost no effect on the oscillations of the grid potential , which occur synchronously with the natural oscillations of the piezoquartz plate. The damping of oscillations of a piezoquartz plate is very small, the damping decrement is less than one ten-thousandth.
In the diagram shown in Fig. 407, feedback is carried out through a capacitor not large capacity C. When generating high-frequency oscillations, the interelectrode capacitance (anode-grid in a generator lamp) often turns out to be sufficient to implement feedback and replaces capacitor C. The resistance prevents large (exceeding the calculated value) negative potentials from appearing on the grid; charges flow down this resistance.
The use of piezoquartz stabilizers makes it possible to maintain the frequency of tube oscillators constant with an accuracy of parts per million. This is used in a piezo quartz clock, which is a tube oscillator with a piezo quartz stabilized oscillation frequency and a device for automatic counting number of completed oscillations. Piezoquartz watches are incomparably more accurate than the best chronometers. They measure time with an accuracy of up to. Using piezoquartz clocks, minor irregularities in the speed of the Earth's daily rotation were discovered and studied.
Rice. 407. Tube oscillation generator with piezoquartz frequency stabilizer
Along with tube generators that create harmonic voltage oscillations, tube generators of voltage pulses that differ sharply in shape from sinusoidal ones are often used. Such so-called relaxation oscillations serve, in particular, to control the electron beam in oscilloscopes and television tubes. Sawtooth-shaped voltage pulses are supplied (in television tubes) to coils that create a magnetic field that deflects the beam, or (in oscilloscopes) to a capacitor, between the plates of which an electron beam passes, which allows for uniform deflections of the beam drawn on the screen over time.
straight line-beam sweep. In Fig. 408 shows a circuit of a tube oscillator that produces sawtooth voltage pulses. Here are two triodes combined in one container, and their grids are connected. It is important that the anode circuit of the first triode (blocking generator) is very strongly connected to the grid circuit through a transformer, which has an iron core to increase mutual inductance. Oscillations in the grid circuit are determined by the appearance of a charge on the capacitor and the flow of this charge through the resistance to the ground; the smaller the time constant of this circuit, the faster the grid capacitor discharges
Rice. 408. Blocking generator and sawtooth voltage pulse generator.
If at the initial moment the grid potential was negative and the blocking oscillator lamp (left triode) was locked, then when the capacitor is discharged, a rapidly increasing current passes through the lamp; this rapid increase in current is ensured by the fact that as the current increases, a positive voltage is supplied to the grid through the transformer (when turning on the transformer windings, the correct polarity must be selected). Further, it is significant that the blocking generator lamp operates in a mode where a large anode current corresponds to a very large leakage of electrons through the grid; Thanks to this grid current, following a positive surge (curve 1 in Fig. 408), the voltage on the grid quickly becomes negative again and the blocking generator lamp is locked again. The voltage at the anode of the second triode (curve 2 in the same figure) drops sharply and deeply every time current begins to pass through the lamp, since high resistance(on the order of When the lamp is locked, the voltage is restored, increasing approximately linearly, and at a faster rate, the shorter the time constant of the anode circuit
It is difficult to set up a modern radio receiver without appropriate measuring equipment. In this case, first of all, a signal generator is needed, i.e. a generator that creates high-frequency oscillations in a certain frequency range. With its help, you can configure high- and intermediate-frequency resonant amplifiers, check the pairing of circuits in a superheterodyne receiver, determine the natural frequency of oscillatory circuits and carry out a number of other measurements.
The schematic diagram of the signal generator is shown in Fig. 1. It consists of a high frequency oscillator, a low frequency oscillator (modulator), a rectifier and an output device. The device allows you to obtain high-frequency modulated or unmodulated oscillations, as well as low-frequency oscillations with a frequency of about 400 Hz. Signal generator frequency range 100 kHz - 16 MHz divided into the following subranges: 100-250 kHz; 250-700 kHz; 700-2000 kHz; 2-5,5 MHz and 5.5-16 MHz The value of the output voltage at the output of the signal generator can reach 0.8-1; it depends on the quality factor of the circuits. The device is powered from the mains AC voltage 127 or 220 V,
The high-frequency generator is made on the left triode of lamp L1 according to a three-point circuit with autotransformer feedback. On each of the subbands, the oscillatory circuit is formed by one of the inductors LI - L5, one of the trimmer capacitors C1-C5 and variable capacitor C7. The transition from one subband to another is carried out using a switch B1. The constant voltage to the lamp's iodine is supplied through a resistor R3. A smooth change in frequency is produced by a variable capacitor C7. The functions of the gridlick are performed by a capacitor C6 and resistors Rl, R2. At high frequency, the lamp's iodine is grounded by a capacitor C8.
The modulator is regular generator audio frequency with capacitive feedback. A conventional inductor Dr1 is used as a loop coil low frequency. The oscillatory circuit of the low-frequency generator is formed by a choke coil Dr1 and capacitors constant capacity SI, C12. The modulator is assembled on the right triode of the lamp L1. To reduce the harmonic content (improving the shape of the low-frequency voltage curve), resistor R12 is included in the cathode of the right triode. The sound generator is turned off using the VZ switch.
Anode modulation is used in the signal generator circuit. Low frequency alternating voltage from the anode of the right triode is supplied to the anode of the left triode simultaneously with the supply voltage through resistor R3. Thanks to the nonlinear processes occurring in the high-frequency generator lamp, the modulation process is carried out.
The output device of the signal generator consists of a smooth divider R2, the scale of which is divided into 10 divisions. To further reduce the output voltage, a step divider formed by resistors R4-R11 is used. Each cell containing two resistors reduces the voltage by 10 times. The necessary attenuation of the signal taken from the smooth divider (sometimes called an attenuator, i.e., weakener) by 1, 10, 100, 1000 and 10,000 times is carried out by switch B2. For example, when setting the switch B2 to position"10-1" ia RF output socket with resistor R5 a voltage is supplied equal to a tenth of the voltage removed from the potentiometer R2; nine-tenths of the last voltage is quenched by a resistor R4, the resistance of which is 9 times greater than the resistance of the right side of the divider between points a-b. Thus, four divider cells make it possible to reduce the voltage by /0 4 times, which, when setting the smooth divider to the position corresponding to 0.1 V, allows you to get the lowest voltage of the order of 10 mkv.
It should be noted that in a signal generator of the simplest type, the amplitude of oscillations across ranges and within each range varies quite a lot, so the use of such dividers allows only indirect judgment of the actual voltage of the signal generator.
Resistor R1 serves to reduce the influence of the signal generator load on the oscillation frequency. On rns. 1 shows the actual resistance values of resistors R4-R11. They are selected from the closest values of resistors produced by our industry -
Low frequency voltage for testing various low frequency amplifying devices is removed from potentiometer RI3 and goes to the LF socket. Resistor R17, being a grid leakage resistance, simultaneously reduces the load response to the operating mode of the low-frequency generator.
The rectifier is mounted according to a conventional single-half-node circuit on two germanium diodes D1 And D2. To reduce the likelihood of diode breakdown, the latter are shunted with resistors R18, R19. Transformer winding switching Tpl for operation from a network with different voltages is carried out by a fuse Ave. The rectifier filter is two-section and consists of capacitors C13, C14 and resistors R15, R16.
The signal generator is mounted on an angular chassis made of 1.5-thick duralumin mm. In order to protect the equipment under test from direct radiation from the generator circuits (in addition to the attenuator), all circuits, the switch and the variable capacitor must be enclosed in a separate shield.
The coils are wound on ceramic frames with a diameter of 10 mm and have SCR-1 type cores for adjustment. Winding coils L1-L4 type suiiversal", winding width 5 mm. Coil L1 contains 850 turns of PELSHO 0.12 wire with a tap from the 200th turn; L2 - 275 turns of PELSHO 0.2 wire with a tap from the 70th turn; L3 -112 turns of lyceidrate wire 7X0.07 with a tap from the 45th turn; L4 - 42 turns of 7X0.07 licendrat wire with a tap from the 15th turn. Coil L5 is single-layer, has 11 turns of ordinary winding, PELSHO wire 0.51 with a tap from the 5th turn. The coils can also be wound on ceresin-impregnated paper or bakelite frames of appropriate sizes. When performing the viaval winding, it is necessary to make the cheeks. The number of turns in this case will differ from those indicated.
Any variable capacitor C7 can be used, but preferably one of frequency, then during calibration you can obtain a uniform placement of divisions on the scale. It is best to use a ceramic range switch.
Throttle Dr1 made using a Ш16 core, set thickness 16 mm. PEL wire 0.15 is wound onto the frame until filled. Almost any inter-lamp transformer can be used.
Transformer Tpl has a core Ш22, thickness of the set is 32 mm. The network winding consists of two sections. Section I contains 763 turns of PEL 0.31 wire, section 11-557 turns of PEL 0.2 wire. Boost winding III contains 1140 turns of PEL 0.2 wire, lamp filament winding IV - 44 turns of PEL 1.0 wire. In this design you can use any power transformer from receivers "Moskvich-V", "Volia", ARZ, etc.
For convenience of working with the device, the rotor of the variable capacitor C7 is rotated using a vernier device, the design of which is easy to understand from Fig. 2.
The front panel of the device has dimensions 210X160 mm. The installation of the main parts was carried out on a horizontal panel measuring 200X120 mm. Depending on the type of parts used, chassis dimensions may vary.
Setting up the device begins with checking the generation, listening to the signal in a known good receiver. To do this, using a piece of coaxial cable, at the end of which there is a special plug, the high-frequency output of the signal generator is connected to the input of the receiver. The presence of generation can also be checked using an avometer operating in the DC voltage measurement mode, which is connected to the anode of the left triode. If, when the control grid of the left triode is short-circuited to the cathode, the voltage at the anode does not drop, the generator operates. Usually, with working parts and the lamp, it starts working immediately.
The operation of the sound generator can be easily checked by applying low-frequency voltage from the output of the skgyal generator to the pickup sockets of the broadcast receiver. The required generation frequency is set by changing the capacitance of capacitors C1I, C12'.
Having established that the high-frequency generator operates in all positions of switch B1 and normal modulation takes place, we begin to adjust the boundaries of individual subranges. The adjustment begins with the long-wave section of the first range (at the maximum capacity of the variable capacitor C7). By rotating the core or changing the number of turns of coil L1, the frequency is set to 100 kHz. Then the tuning knob is moved to the other extreme position (corresponding to the minimum capacitance of capacitor C7) and the frequency of the generator is determined. If it is higher than required, increase the capacitance of the tuning capacitor C/ and repeat the adjustment again. To set the boundaries of the second subrange, capacitor C7 is also set to the position of maximum capacitance and by selecting the inductance of the coil L2, it is ensured that at the beginning of the scale of this subrange the generator frequency is slightly lower than the frequency (250 kHz) at the same scale of the first subrange. The boundaries of the remaining subranges are set in a similar way. Calibration of the SG is carried out according to the generally accepted method - using the GSS using the beat method, using a control receiver or a heterodyne resonance indicator - GIR.
device, circuit diagram which is shown in Fig. 1, represents sound generator, operating in the frequency range from 23 Hz up to 32 kHz. The entire frequency range is divided into four subranges 23-155 Hz, 142-980 Hz, 800-5500 Hz, 4.9-32 kHz. The device has an output voltage indicator, as well as smooth and step dividers, with which you can adjust the output voltage from 10 mV up to 10 V. Coefficient nonlinear distortion does not exceed 3%. Output voltage measurement accuracy 3%.
As can be seen from Fig. 1, the sound generator consists of a two-stage exciter JI1, cathode follower L2, output device and rectifier.
The exciter is assembled according to a circuit with rheostatic-capacitive tuning and is a two-stage low-frequency amplifier with positive feedback. The first amplification stage is assembled on the left triode of lamp L1 with a load in the form of a resistor R17. The second amplification stage is assembled on the right triode of the lamp L1. A resistor is used as a load R18. Communication between the stages is carried out through the capacitor Sat. The positive feedback necessary for the occurrence of oscillations is supplied from the anode circuit of the right triode to the control grid of the left triode through a large capacitor C5 and a divider consisting of two sections: a resistor R14, series connected capacitors C1, C2 and resistor R7 and capacitors connected in parallel NW, C4. The voltage acting on the control grid of the left triode L1, removed from the parallel section of the divider R7. NW, C4. The use of a frequency-dependent divider makes it possible to obtain self-excitation conditions only for one frequency at which the phase shift between the positive feedback voltage and the control grid of the left triode (divider R7, SZ, C4) and the anode of the right triode L1 equal to July. This makes it possible to obtain sinusoidal oscillations using such a generator.
To change the generation frequency, it is necessary to change the parameters of the elements included in the divider chains. In this circuit, a smooth change in frequency is carried out by changing the capacitance of the dual capacitor CI, C4, and jumping - with a switch B1, which changes the values of the resistors included in the divider chains ( R5, R6 And R12, R13; R3, R4 And R10, Rll; Rl, R2 And R8, R9).
As calculations show, at any frequency there will always be enough supply to the control grid of the left triode of lamp L1 high voltage, therefore, the amplifier stages will introduce large distortions due to overload. These distortions are reduced using negative feedback, the circuit of which consists of a variable resistor R15, constant resistor R16 and incandescent lamps included in the left cathode of the lamp LZ, L4. The negative feedback circuit also stabilizes the output voltage, which changes relatively strongly when the frequency changes. As the exciter output voltage increases, the depth of the negative feedback increases, reducing the gain of the first stage of the generator. Thus, the output voltage of the generator will be stabilized over the range. The least distortion at the exciter output will be when the voltage removed from the parallel branch of the divider is close to the negative feedback voltage, the value of which, when adjusting the device, is set using a variable resistor R15.
From the exciter output through a transition capacitor C7 audio frequency voltage is supplied to the input of the cathode follower assembled in the lamp L2. The lamp load is a potentiometer R23. Divider consisting of resistors R22, R21, the required operating mode of this cascade is established. Resistor R20 is restrictive. The use of a cathode follower, which has a high input resistance, makes it possible to reduce the load response to the generator frequency and the amount of distortion introduced by the output stage.
The output device consists of a smooth(R23) and stepped(R26, R27; R28, R29) dividers and a conventional diode voltmeter, which uses a galvanometer with a scale of 50 mka. Resistors R24, R25 installation Application of resistor R30 allows for better scale linearity.
The rectifier is assembled using a conventional full-wave voltage doubling circuit. The device can be powered from an alternating current network with a voltage of 110, 127 and 220 e.
Switch B1 two-plate and four positions. The second board is used to attach individual resistors of the frequency-dependent divider.
Lamps LZ, L4 used from the film projector "Luch" (110 V, 8 Tue). You can use one lamp IA 220 V power 10-25 Tue. Power transformer from the Record-53M receiver. You can also use transformers from Moskvich-V, Volna, ARZ-52, etc. receivers.
For the convenience of setting up the device, the branches of the frequency-dependent divider are made up of two series-connected resistors (Rl, R2, R8, R9, etc.). Setting up the generator begins with checking the operation of the rectifier. Under load, the voltage at the rectifier output should be 280-320 A. The current consumed by the device from the rectifier should be within 30-35 mA. After this, to the generator output (///- Gn1) - connect an oscilloscope and achieve stable oscillations and absence of distortion in the lowest frequency subrange. The shape of the generated oscillation curve is greatly influenced by the magnitude of the negative feedback. With weak negative feedback (R15 is large), more stable oscillations are obtained, but with noticeable shape distortions. When the connection is strong, the oscillations are disrupted. Therefore, by selecting the value of negative feedback (R15), a compromise solution is found: the feedback depth is chosen such that it ensures sufficiently stable generation over the entire frequency range and good shape crooked.
To calibrate the generator scale, you can use a frequency meter or generator audio frequencies. In the latter case, the calibration of each of the four scales is carried out using Lissajous figures observed on the screen of the oscilloscope tube. The output indicator is calibrated using a standard lamp voltmeter, which is connected between points a-b schemes. Changing the voltage supplied to the input of the divider (or indicator) is carried out by a potentiometer R23, and in which an alternating voltage component of the order of 13 is released V. By setting the voltage on a standard voltmeter 10 V variable resistor R24, make sure that the indicator needle deviates to the full scale. Setting on a standard voltmeter with potentiometer R23 voltage corresponding to 9, 8, 7, 6, 5, 4, 3, 2 and 1 V, Each time they make the appropriate notes on the CA indicator scale.
It should be noted that the presence of permanent capacity C2 in the upper branch of the divider significantly improves the conditions for the occurrence of oscillations at high frequencies and helps to equalize the amplitude of exciter oscillations at any position of the block of variable capacitors. If a 6P14P lamp is missing, it can be replaced with lamps of the 6P15P, 6P18P or 6Zh5P type.
Stable band generator in amateur radio practice, the number one problem is still the stability of the frequency of generators with smooth tuning. Every shortwave operator knows how unpleasant and sometimes difficult it is to work with a correspondent when the frequency of his transmitter “creeps” up or down. This is especially noticeable when operating CW or SSB. But besides the subjective factor, there is also an official regulation that strictly determines the stability of the frequency of a shortwave radio station. The drift of the generator frequency in amateur radio practice is not always caused by the negligence of the designer-operator: people of different ages and professions with varying degrees of special training are engaged in working on short waves.
In laboratory conditions, as a result of analysis and numerous experiments, a circuit for setting a stable range oscillator was selected, which is offered to the attention of readers. This generator can also be used as a local oscillator in a receiver, in measuring equipment, etc. When choosing a generator circuit, a number of curves were considered that characterize the frequency shift depending on changes in the supply voltage various schemes tube oscillators, the circuit described below has the greatest stability. The remaining factors affecting the frequency stability of the tube oscillator are taken into account and compensated by known methods. Obviously, it will be more convenient to trace this directly on the proposed circuit (Fig.).
The whole contains three stages: the generator itself on a 6N15P lamp (L1), a cathode follower and an amplifier on a 6F1P lamp (L2).
Actually stable range generator
assembled according to a circuit with negative resistance. The operation of generators with negative resistance is quite fully covered in the literature (for example, see A. A. Kulikovsky “New in Amateur Radio Reception Technology”, Thomas Martin “Electronic Circuits”). In essence, the circuit is an asymmetrical multivibrator, in one of the circuits of which a reactive element is included. Direct communication between the generator triodes is carried out through -tod; the positive feedback necessary for generation to occur is from the anode of the right (according to the circuit) triode to the grid of the left triode.
Here it is necessary to dwell on one very significant detail, not emphasized in the literature. This detail mainly affects the operation of the generator and which many designers did not pay attention to and were forced to abandon it.
The point is that, as noted above, direct communication between the generator triodes is carried out through the cathode. Thus, the cathode load will be a load for both alternating and direct current. What happens if there is only active resistance in the cathode? First of all, the value of this resistance will be selected to ensure desired mode cascade.
In practice, its value will not exceed 2-3 lumps. In turn, this resistance is also a load for high-frequency voltage. And here, as a rule, it turns out that its value is too small and does not provide sufficient transfer of RF energy to the right triode in the circuit. In addition, this resistance significantly shunts the generator circuit, greatly reducing its quality factor, worsening the already difficult excitation conditions. Having analyzed the circuit of a stable band generator in this way, you can come to a simple solution: turn on the RF choke in series with the cathode resistance of the load. Now the complex cathode load will add up over the DC current.
In the general case, the capacitance of capacitor C1 can be selected within a few picofarads. The generation turns out to be so stable that when the anode voltage decreases to 10 V, an RF voltage of about 1.5 V remains at the cathode choke. Returning to the specific data of the above circuit, we note that the positive change in the capacitance of the generator circuit from heating during operation is compensated by capacitor C3 (KTK blue). Capacitor C3 must be KSO-2 group “G”. Capacitor C1 - type KTK blue.
To further increase stability, it is advisable to remove the HF voltage to the next stage precisely from the cathode load inductor, and not from any other point in the circuit, for the following reasons: by removing the HF voltage directly from the generator circuit, from the anode of the right triode, or directly from the generator cathode, we violate vibration stability. By removing the signal from the cathode choke, we almost completely isolate the generator.
Here it is especially clear how justified this particular sequence of connecting the resistance and inductor to the cathode of the generator is. In fact, the cathode load circuit in our case for HF can be represented as a divider consisting of two series resistances: R1, which, depending on the type of lamp and the selected generator mode, can be from several ohms to 2-3 kohms; and reactance of the inductor Rx, which is best case scenario disproportionately large compared to R1 (Fig.) 
Thus, for an RF signal, the value of R1 in our divider turns out to be very small, and we can assume that in the best case, in terms of HF, Uin will be equal to Uout, or, in other words, the RF voltage removed from the inductor will be equal to the RF voltage at the generator cathode. However, in real conditions, of course, the RF resistance of the choke will have a specific value due to the final parameters of the latter and the influence of the circuit as a whole.
But nevertheless, its value will be much greater than R1 and the loss in the voltage removed will be insignificant. At the same time, resistance R1 protects to a large extent from possible interference in the communication circuit that ensures the operation of the generator. To further “decouple” the stable range generator from subsequent stages, there is a buffer stage assembled according to the cathode follower circuit on the L2 lamp triode. As is known, the cathode follower has a high input resistance and practically does not bypass the inductor Dr1. It is necessary to note one more advantage of this generator.
When selected accordingly, it has a small percentage of harmonics. In most cases, even the second harmonic could not be measured. This is quite positive quality, especially when using such an oscillator as a local oscillator in a multi-converter receiver or as a VFO in an SSB transmitter, where there is a risk of combination frequencies or interference whistles.
However, in the described stable band generator, we mean further multiplication of the frequency to obtain all amateur bands; for this purpose, after the cathode follower, there is an amplifier stage at the main frequency (80 m amateur band), assembled on the pentode part of the L2 lamp. To measure the frequency drift of the generator, a decade counter ECh-1 was used, since, for example, the 526U wave meter was not able to measure the frequency drift at all during an hourly test. The main measurement was taken after a twenty-minute warm-up. The frequency drift during the first 15 minutes of measurement was: 3,645,282-3,645,245 Hz-37 Hz! Over the next 15 minutes the frequency drift was 33 Hz.
It should be noted that during the experiment only the anode voltage was stabilized. The screen of the master oscillator circuit (L1) was located near the generator lamp screen at a distance of 22 mm. The circuit was deliberately chosen with a low quality factor Q = 60. It had 60 turns of PE 0.29 wire, wound turn to turn on a polystyrene frame with a diameter of 8 mm, and was enclosed in a brass screen with a diameter of 21 mm (coil L2 is wound on the same frame with with the same screen configured with a ferrite core and had 37 turns of PELSHKO 0.2 wire, winding “universal”, winding width 4 mm). It can be argued that if additional measures are taken; stabilize the filament of the generator lamp with a barretor, use a master oscillator circuit with a high quality factor, isolate the generator circuit thermally as best as possible, then the stability will be even higher.
In conclusion, let us dwell on the method of manipulation used here. The manipulation is carried out not by disrupting generation, as usual, but by shifting the frequency to the side, beyond the transmission limits of the transmitter circuits. This is carried out by a miniature relay RES-10 (it is possible to use a relay RES-9), which has dimensions of 10X 16 X 19 mm, weighs 7.5 g, operates at temperatures up to +125 ° C and relative humidity up to 98%. At the same time, it is low-capacity and has a response time of 5 ms. This relay and the manipulation process connects a stable band generator capacitor Ca to the circuit, moving the generator frequency to the side, but without disrupting it.
The test was carried out subjectively using a 526U wave meter. During the manipulation, not the slightest “squelching” or any other undesirable phenomena were noticed. There are no clicks at all. The experiment carried out allows us to assert that such a manipulation method can be recommended to shortwave operators as simple, high-quality and very effective.
In Volume II, § 106, we became acquainted with the structure of a vacuum tube and saw that changing the voltage on its grid changes the current in its anode circuit. When the grid is negatively charged, electrons cannot fly to the anode, no current flows, and the lamp is, as they say, “locked.” By charging the grid positively, we “unlock” the lamp, i.e. current can flow through it. Changes in the anode current follow changes in the voltage on the grid almost instantly - after ten billionths of a second (the time of flight of electrons from the grid to the anode), i.e. vacuum tube is a “switch” with negligible inertia. Therefore, by connecting a lamp with an oscillating circuit and a battery so that at the right moments the lamp is unlocked and passes current to the capacitor, we can obtain an electrical self-oscillating system that allows us to excite (generate) undamped electrical oscillations.
Obviously, in order for oscillations in the circuit to control the anode current of the lamp, it is necessary to apply a voltage to its grid, depending on fluctuations in the current or voltage in the circuit, i.e., as they say, connect the circuit with the grid circuit of the lamp. This electrical connection can be made in various ways- using electrostatic induction (capacitive coupling), using electromagnetic induction (inductive coupling), etc. The main thing here is not exactly how the circuit is connected to the lamp, but that thanks to this connection we have not only the effect of the lamp on vibrations in the circuit, but also the reverse effect of these vibrations on the lamp. Various methods of connecting a lamp to an oscillating circuit that provide such a feedback effect are examples of the so-called feedback, and electric self-oscillating systems of this kind themselves are called lamp generators. Modern tube generators make it possible to obtain oscillations with frequencies of up to several billion hertz and are used extremely widely. They form the basis of every radio and are found in many types of radios.
In Fig. Figure 58 shows one of the very numerous and varied tube oscillator circuits - a circuit with inductive feedback.
An oscillatory circuit, consisting of an inductor and a capacitor, is connected in series with the battery in the anode circuit of the lamp, i.e., between the anode and the heated filament (cathode). The filament is heated by current from a filament battery. In the grid circuit of the lamp - between the grid and the cathode - there is a second inductor connected inductively to the circuit coil. Thus, the coils form, as it were, the primary and secondary windings of the transformer, but without a core. However, in low (sound) frequency generators you can use a transformer with an iron core.
The coil controls the voltage on the grid and provides feedback between oscillations in the circuit and on the lamp grid.
Let's imagine that oscillations occur in a circuit consisting of an inductor and a capacitor. An alternating current flows through the coil, which induces an alternating current in the coil. d.s. The grid is charged either positively or negatively with respect to the cathode, and the period of these oscillations of the grid voltage is obviously the same as the period of oscillations in the circuit, i.e.
The lamp is either “unlocked” or “locked”; Thus, oscillations in the circuit cause pulsations in the anode current of the lamp. The anode current coming from the anode through the circuit to the cathode, branching, passes through the inductor and capacitor (of course, the constant, i.e., not changing over time, component of the anode current passes only through the coil, since D.C. cannot go through a capacitor, see Volume II, § 159). If the phase of the oscillations of the anode current is selected correctly, that is, the “jokes” of the anode current act on the circuit at the right moments, then the oscillations in the circuit will be maintained (cf. § 30). In other words, for each period of oscillation, a portion of energy will be borrowed from the battery, just covering the energy losses in the circuit during the same time, and the oscillations will be undamped. If you swap the ends of the coil, then the phase of the grid voltage oscillations will change by 180°, and the oscillations will not be excited (similar to what happened in the system shown in Fig. 56).
Rice. 58. Tube generator
Oscillations can be observed using an electronic oscilloscope or, if the oscillations have an audio frequency, using a loudspeaker connected directly to the anode circuit of the lamp. You can also include an incandescent light bulb (from a flashlight or car, depending on the power of the generator) into the capacitor branch of the circuit. Since the light bulb is connected in series with the capacitor, the constant component of the anode current does not pass through it. Consequently, the light bulb will light up only if there are electrical oscillations in the circuit.
Using a tube generator similar to the one described, it is not difficult to observe the phenomenon electrical resonance, connecting inductively with the generator circuit a second similar oscillating circuit, but with a variable capacitor and with an incandescent light bulb included in the circuit. By smoothly changing the capacitance in this circuit, it can be tuned to resonance at the frequency of the generator. With the appropriate selection of the light bulb and the connection between the circuits, it is not difficult to achieve such conditions that the light bulb flashes when there is resonance, and goes out when out of tune.
Devices and accessories: three-electrode lamp, source DC voltage at 300 V, source AC voltage at 4V, two air capacitors of constant and variable capacitance, two inductance coils, two capacitors of fixed capacitance, resistance, microammeter, high-frequency electromagnetic field indicator on a neon lamp, unknown capacitance and inductance.
An electrical oscillatory circuit is a circuit (Fig. 1) consisting of series-connected capacitance C, inductance L and resistance R of conductors.
Periodic changes in current strength and related quantities occur in the circuit. The recharging of the capacitor plates can be understood by remembering what the phenomenon of self-induction consists of.
The phenomenon of self-induction is as follows: with any change in the current in the circuit, an emf appears in it. self-induction c, which is directly proportional to the rate of change of current in the circuit (di/dt) and inversely to this speed is directed:
If the current increases, the emf. prevents this increase in current and creates an induced current in the opposite direction. If the current decreases, the emf. prevents the current from decreasing and creates an induced current in the same direction.
Let's consider the operation of the circuit. Let's charge the capacitor from external source electricity to a certain potential difference U, imparting charges to its plates ±q, and then using key K to close the circuit, the capacitor will begin to discharge and some current will flow in the circuit. At a low R value it will increase very quickly. Direction for current i, shown in Fig. 1, we will take it as positive (the upper plate is charged positively, the lower one - negatively) and consider the processes occurring in the circuit.
Let us first assume that the ohmic resistance of the conductor that makes up the circuit is vanishingly small, i.e. R»0, and let at the initial moment of time the charge of the capacitor is maximum ( q=q o). In this case, the potential difference between its plates is also maximum (U = U o), and the current in the circuit is zero (Fig. 2, a). When the capacitor begins to discharge, current will flow in the circuit.
As a result, energy electric field will decrease, but an ever-increasing energy of the magnetic field will arise due to the current flowing through the inductance. Since the emf acts in the circuit. self-induction, the current will increase gradually, and after a time t=1/4 T (a quarter of a period) it will reach maximum value (i=i o), the capacitor will be completely discharged and the electric field will disappear, i.e. q=0 and U=0. Now all the energy of the circuit is concentrated in the magnetic field of the coil (Fig. 2, b). At the next moment of time, the magnetic field of the coil will begin to weaken, and therefore a current is induced in it, flowing (according to Lenz’s rule) in the same direction in which the discharge current of the capacitor went. Thanks to this, the capacitor is recharged. After time t=1/2 T, the magnetic field will disappear, and the electric field will reach its maximum. At the same time q=q o , U=U o and i=0. Thus, the energy of the magnetic field of the inductor will be converted into the energy of the electric field of the capacitor (Fig. 2, c). After a time t=3/4 T, the capacitor will be completely discharged, the current will again reach its maximum value (i=i o), and the energy of the circuit will be concentrated in the magnetic field of the coil (Fig. 2d). At a subsequent moment in time, the magnetic field of the coil will begin to weaken and the induction current, preventing this weakening, will recharge the capacitor. As a result, by time t=T the system (circuit) returns to initial state(Fig. 2, a) and the repetition of the considered process begins.
During the process, the charge and voltage on the capacitor, as well as the strength and direction of the current flowing through the inductance, periodically change (oscillate). These oscillations are accompanied by mutual transformations of the energies of the electric and magnetic fields.
Thus, if the circuit resistance is zero, then the specified process will continue indefinitely and we get undamped electrical oscillations, the period of which will depend on the values of L and C.
Oscillations occurring in such an ideal circuit (R = 0) are called free, or own, circuit oscillations with a period
. (10)
In a real oscillatory circuit, the ohmic resistance R cannot be reduced to zero. Therefore, electrical oscillations in it will always be damped, since part of the energy will be spent on heating the conductors (Joule heat).
To implement undamped electrical oscillations, it is necessary to ensure automatic feeding energy with a frequency equal to the frequency of natural oscillations of the circuit, i.e. it is necessary to create a self-oscillating system. Such a system of continuous oscillations is a tube oscillator.
Tube generator
The simplest scheme tube generator of continuous electromagnetic oscillations is shown in Fig. 3
It consists of an LC oscillatory circuit connected to the anode circuit of a three-electrode lamp in series with a constant anode voltage source B A. The anode battery B A is like a “reservoir” from which energy is supplied to the oscillatory circuit. Coil L 1 is inductively coupled to coil L of the circuit, the ends of which are connected to the grid and cathode of the lamp. It connects the operation of the lamp with the oscillatory process in the circuit and is called a feedback coil.
The three-electrode lamp, together with the feedback coil, serves to ensure that energy is supplied to the circuit in time with the oscillations. Undamped oscillations are obtained due to periodic recharging of the capacitor by the anode current of the lamp passing through the circuit. In order to periodically recharge the circuit capacitor at the required times, the anode current must be pulsating. This is ensured by a corresponding change in the potential on the lamp grid, which changes when the direction of the discharge current in the LC circuit changes due to the phenomenon of mutual induction between the coils L and L 1.
If there is a negative charge on the grid, the lamp is “locked”; the anode current will not flow through the lamp. The oscillating circuit will operate at normal mode. When there is a positive charge on the grid, the lamp will “open” and recharge the capacitor. The process will then begin to repeat.
Thus, the lamp periodically supplies energy from the anode battery to the circuit. Due to this, undamped electrical oscillations occur in the circuit.