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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Supergenerator. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / radio reception What is a super-regenerator, how does it work, what are its advantages and disadvantages, in which amateur radio designs can it be used? This article is devoted to these questions. A super-regenerator (it is also called a super-regenerator) is a very special type of amplifying, or amplifying-detector device, which, with exceptional simplicity, has unique properties, in particular, a voltage gain of up to 105 ... 106, i.e. reaching a million! This means that sub-microvolt input signals can be amplified to fractions of a volt. Of course, it is impossible to obtain such amplification in one stage in the usual way, but a completely different method of amplification is used in the superregenerator. If the author is allowed to philosophize a little, then we can not quite strictly say that the super-regenerative amplification occurs in other physical coordinates. Conventional amplification is carried out continuously in time, and the input and output of the amplifier (four-terminal), as a rule, are separated in space. This does not apply to two-terminal amplifiers, such as a regenerator. Regenerative amplification occurs in the same oscillatory circuit to which the input signal is applied, but again continuously in time. The super-regenerator works with samples of the input signal taken at certain points in time. Then the sampling is amplified in time, and after a certain period the amplified output signal is taken, often even from the same terminals or sockets to which the input is also connected. While the amplification process is in progress, the superregenerator does not respond to input signals, and the next sample is taken only when all amplification processes are completed. It is this amplification principle that makes it possible to obtain huge coefficients, the input and output do not need to be decoupled or shielded - after all, the input and output signals are separated in time, therefore they cannot interact. The super-regenerative method of amplification also has a fundamental drawback. In accordance with the Kotelnikov-Nyquist theorem, for undistorted transmission of the signal envelope (modulating frequencies), the sampling frequency must be at least twice the highest modulation frequency. In the case of an AM broadcast signal, the highest modulating frequency is 10 kHz, an FM signal is 15 kHz, and the sampling frequency must be at least 20 ... 30 kHz (we are not talking about stereo). The bandwidth of the super-regenerator is obtained in this case by almost an order of magnitude greater, i.e., 200...300 kHz. This disadvantage cannot be eliminated when receiving AM signals and was one of the main reasons for superregenerators to be replaced by more advanced, albeit more complex, superheterodyne receivers, in which the bandwidth is equal to twice the highest modulating frequency. Strange as it may seem, in FM the described disadvantage manifests itself to a much lesser extent. FM demodulation occurs on the slope of the resonant curve of the superregenerator - FM is converted to AM and then detected. In this case, the width of the resonant curve should not be less than twice the frequency deviation (100...150 kHz), and a much better matching of the bandwidth with the width of the signal spectrum is obtained. Previously, super-regenerators were made on vacuum tubes and became widely used in the middle of the last century. Then there were few radio stations on the VHF band, and a wide bandwidth was not considered a particular disadvantage, in some cases even facilitating tuning and searching for rare stations. Then superregenerators on transistors appeared. Now they are used in radio control systems for models, burglar alarms, and only occasionally in radio receivers. Super-regenerator circuits differ little from regenerator circuits: if the latter periodically increases the feedback to the generation threshold, and then reduces it until the oscillations stall, then a super-regenerator is obtained. Auxiliary damping oscillations with a frequency of 20 ... 50 kHz, periodically changing the feedback, are obtained either from a separate generator, or occur in the high-frequency device itself (super-regenerator with self-quenching). Basic scheme of the regenerator-super-regenerator For a better understanding of the processes occurring in the superregenerator, let us turn to the device shown in Fig. 1, which, depending on the time constant of the R1C2 chain, can be both a regenerator and a super-regenerator.
This scheme was developed as a result of numerous experiments and, as it seems to the author, is optimal in terms of simplicity, ease of adjustment and the results obtained. Transistor VT1 is connected according to the oscillator circuit - an inductive three-point. The generator circuit is formed by a coil L1 and a capacitor C1, the coil tap is made closer to the base terminal. Thus, the high output resistance of the transistor (collector circuit) is matched with a lower input resistance (base circuit). The power supply circuit of the transistor is somewhat unusual - the constant voltage at its base is equal to the collector voltage. A transistor, especially a silicon one, may well operate in this mode, because it opens at a base voltage (relative to the emitter) of about 0,5 V, and the collector-emitter saturation voltage is, depending on the type of transistor, 0,2 ... 0,4 .1 V. In this circuit, both the collector and the DC base are connected to a common wire, and power is supplied through the emitter circuit through resistor RXNUMX. In this case, the voltage at the emitter is automatically stabilized at a level of 0,5 V - the transistor works like a zener diode with the specified stabilization voltage. Indeed, if the voltage at the emitter drops, the transistor closes, the emitter current decreases, and then the voltage drop across the resistor decreases, which will lead to an increase in the emitter voltage. If it increases, the transistor will open more and the increased voltage drop across the resistor will compensate for this increase. The only condition for the correct operation of the device is that the supply voltage should be noticeably higher - from 1,2 V and higher. Then the transistor current can be set by selecting the resistor R1. Consider the operation of the device at high frequency. The voltage from the lower (according to the scheme) part of the turns of the coil L1 is applied to the base-emitter junction of the transistor VT1 and is amplified by it. Capacitor C2 is a blocking capacitor, for high frequency currents it presents low resistance. The load in the collector circuit is the resonant resistance of the circuit, somewhat reduced due to the transformation of the upper part of the coil winding. When amplifying, the transistor inverts the phase of the signal, then it is inverted by the transformer formed by parts of the coil L1 - the phase balance is performed. And the balance of amplitudes necessary for self-excitation is obtained with sufficient amplification of the transistor. The latter depends on the emitter current, and it is very easy to adjust it by changing the resistance of the resistor R1, by including, for example, two resistors in series instead of it, a constant and a variable. The device has a number of advantages, which include simplicity of design, ease of installation and high efficiency: the transistor consumes exactly as much current as is necessary for sufficient signal amplification. The approach to the generation threshold turns out to be very smooth, moreover, the adjustment takes place in the low-frequency circuit, and the regulator can be taken from the circuit to a convenient place. The adjustment has little effect on the tuning frequency of the circuit, since the supply voltage of the transistor remains constant (0,5 V), and, consequently, the interelectrode capacitances hardly change. The described regenerator is capable of increasing the quality factor of the circuits in any wave range, from LW to VHF, and the coil L1 does not have to be a circuit coil - it is permissible to use a coupling coil with another circuit (capacitor C1 is not needed in this case). It is possible to wind such a coil on the rod of the magnetic antenna of the DV-SV receiver, and the number of turns should be only 10-20% of the number of turns of the contour coil, the Q-multiplier on a bipolar transistor is cheaper and simpler than on a field one. The regenerator is also suitable for the KB range, if you connect the antenna to the L1C1 circuit with either a coupling coil or a low-capacity capacitor (up to fractions of a picofarad). A low-frequency signal is taken from the emitter of the transistor VT1 and fed through a decoupling capacitor with a capacity of 0,1 ... 0,5 microfarads to the AF amplifier. When receiving AM stations, such a receiver provided a sensitivity of 10 ... 30 μV (feedback below the generation threshold), and when receiving telegraph stations on beats (feedback above the threshold) - units of microvolts. The processes of rise and fall of oscillations But back to the super-regenerator. Let the supply voltage to the described device be applied in the form of a pulse at time t0, as shown in Fig. 2 on top. Even if the transistor amplification and feedback are sufficient for generation, oscillations in the circuit will not occur immediately, but will grow exponentially for some time τn. According to the same law, the decay of oscillations occurs after the power is turned off, the decay time is designated as τs.
In general terms, the law of rise and fall of fluctuations is expressed by the formula Ukont = U0exp(-rt/2L), where U0 is the voltage in the circuit from which the process began; r is the equivalent loss resistance in the circuit; L is its inductance; t - current time. Everything is simple in the case of a decline in oscillations, when r = rp (loss resistance of the circuit itself, Fig. 3).
The situation is different with increasing oscillations: the transistor introduces negative resistance into the circuit - roc (feedback compensates for losses), and the total equivalent resistance becomes negative. The minus sign in the exponent disappears, and the growth law will be written: Ukont = Uсexp(rt/2L), where r = rос - rп From the above formula, one can also find the oscillation rise time, given that the growth starts from the signal amplitude in the circuit Uc and continues only up to the amplitude U0, then the transistor enters the limiting mode, its gain decreases and the oscillation amplitude stabilizes: τн = (2L/r) log(U0/Uc). As you can see, the rise time is proportional to the logarithm of the reciprocal of the received signal level in the loop. The larger the signal, the shorter the rise time. If power pulses are applied to the super-regenerator periodically, with a superization (quenching) frequency of 20...50 kHz, then flashes of oscillations will occur in the circuit (Fig. 4), the duration of which depends on the signal amplitude - the shorter the rise time, the longer the flash duration . If flares are detected, the output will be a demodulated signal proportional to the average value of the flare envelope.
The gain of the transistor itself can be small (units, tens), sufficient only for self-excitation of oscillations, while the gain of the entire superregenerator, equal to the ratio of the amplitude of the demodulated output signal to the amplitude of the input, is very large. The described mode of operation of the super-regenerator is called non-linear, or logarithmic, since the output signal is proportional to the logarithm of the input. This introduces some non-linear distortions, but it also plays a useful role - the sensitivity of the super-regenerator to weak signals is greater, and less to strong ones - here, as it were, a natural AGC operates. For completeness of the description, it must be said that the linear mode of operation of the superregenerator is also possible if the duration of the supply pulse (see Fig. 2) is less than the rise time of the oscillations. The latter will not have time to grow to the maximum amplitude, and the transistor will not enter the limiting mode. Then the flash amplitude will become directly proportional to the signal amplitude. Such a mode, however, is unstable - the slightest change in the transistor gain or the equivalent circuit resistance r will either lead to a sharp drop in the flash amplitude and, consequently, the superregenerator gain, or the device will enter a nonlinear mode. For this reason, the linear mode of the super-regenerator is rarely used. It should also be noted that it is absolutely not necessary to switch the supply voltage in order to get oscillation flashes. With equal success, you can apply an auxiliary superization voltage to the lamp grid, the base or the gate of the transistor, modulating their gain, and hence the feedback. The rectangular shape of the damping oscillations is also not optimal, a sinusoidal one is preferable, and even better a sawtooth one with a gentle rise and a sharp fall. In the latter version, the super-regenerator smoothly approaches the point of oscillation, the bandwidth narrows somewhat, and gain appears due to regeneration. The resulting fluctuations grow slowly at first, then faster and faster. The decline in fluctuations is obtained as quickly as possible. The most widely used are super-regenerators with auto-superization, or with self-extinguishing, which do not have a separate generator of auxiliary oscillations. They only work in non-linear mode. Self-quenching, in other words, intermittent generation, is easy to obtain in a device made according to the scheme of Fig. 1, it is only necessary that the time constant of the chain R1C2 be greater than the rise time of the oscillations. Then the following will happen: the oscillations that have arisen will cause an increase in the current through the transistor, but the oscillations will be maintained for some time by the charge of the capacitor C2. When it is used up, the voltage at the emitter will drop, the transistor will close and the oscillations will stop. Capacitor C2 will begin to charge relatively slowly from the power source through resistor R1 until the transistor opens and a new flash occurs. Stress diagrams in the super-regenerator Oscillograms of voltages at the emitter of the transistor and in the circuit are shown in fig. 4 as they would normally be seen on a wideband oscilloscope screen. The voltage levels of 0,5 and 0,4 V are shown quite conditionally - they depend on the type of transistor used and its mode. What will happen when an external signal enters the circuit, because the duration of the flash is now determined by the charge of the capacitor C2 and, therefore, is constant? With the growth of the signal, as before, the rise time of oscillations decreases, flashes follow more often. If they are detected by a separate detector, then the average signal level will increase in proportion to the logarithm of the input signal. But the role of the detector is successfully performed by the transistor VT1 itself (see Fig. 1) - the average voltage level at the emitter drops with increasing signal. Finally, what happens in the absence of a signal? Everything is the same, only the increase in the oscillation amplitude of each flash will start from a random noise voltage in the superregenerator circuit. In this case, the flash frequency is minimal, but unstable - the repetition period changes chaotically. At the same time, the amplification of the super-regenerator is maximum, and a lot of noise is heard in the phones or the loudspeaker. It decreases sharply when tuned to the frequency of the signal. Thus, the sensitivity of the super-regenerator is very high by the very principle of its operation - it is determined by the level of internal noise. Additional information on the theory of super-regenerative reception is given in [1,2]. VHF FM receiver with low voltage power supply And now let's consider practical schemes of super-regenerators. There are quite a lot of them in the literature, especially of ancient years. An interesting example: a description of a super-regenerator made with only one transistor was published in the magazine "Popular Electronics" No. 3, 1968, its brief translation is given in [3]. The relatively high supply voltage (9 V) provides a large amplitude of oscillation bursts in the superregenerator circuit, and, consequently, a large amplification. This solution also has a significant drawback: the superregenerator radiates strongly, since the antenna is connected directly to the circuit by a coupling coil. It is recommended to turn on such a receiver only somewhere in nature, away from populated areas. A diagram of a simple VHF FM receiver with a low-voltage supply, developed by the author on the basis of the basic circuit (see Fig. 1), is shown in fig. 5. The antenna in the receiver is the loop coil L1 itself, made in the form of a single-turn frame made of thick copper wire (PEL 1,5 and higher). Frame diameter 90 mm. The circuit is tuned to the signal frequency with a variable capacitor (KPI) C1. Due to the fact that it is difficult to make a tap from the frame, the transistor VT1 is connected according to the capacitive three-point circuit - the OS voltage is supplied to the emitter from the capacitive divider C2C3. The superization frequency is determined by the total resistance of the resistors R1-R3 and the capacitance of the capacitor C4. If it is reduced to a few hundred picofarads, intermittent generation stops and the device becomes a regenerative receiver. If desired, you can install a switch, and the capacitor C4 can be made up of two, for example, with a capacity of 470 pF with 0,047 microfarads connected in parallel. Then the receiver, depending on the reception conditions, can be used in both modes. Regenerative mode provides cleaner and better reception, with less noise, but requires significantly higher field strengths. Feedback is regulated by a variable resistor R2, the handle of which (as well as the tuning knob) is recommended to be brought to the front panel of the receiver housing. The radiation of this receiver in the super-regenerative mode is attenuated for the following reasons: the amplitude of oscillation bursts in the circuit is small, on the order of a tenth of a volt, and the small loop antenna radiates extremely inefficiently, having a low efficiency in the transmission mode. The receiver's AF amplifier is two-stage, assembled according to a direct-coupled circuit on transistors VT2 and VT3 of different structures. The collector circuit of the output transistor includes low-resistance headphones (or one telephone) of types TM-2, TM-4, TM-6 or TK-67-NT with a resistance of 50-200 Ohm. Phones from the player will do.
The necessary bias to the base of the first UZCH transistor is supplied not from the power source, but through the resistor R4 from the emitter circuit of the transistor VT1, where, as mentioned, there is a stable voltage of about 0,5 V. The capacitor C5 passes the oscillations of the AF to the base of the transistor VT2. The ripples of the quenching frequency of 30 ... 60 kHz at the input of the ultrasonic frequency converter are not filtered, so the amplifier operates as if in a pulsed mode - the output transistor closes completely and opens to saturation. The ultrasonic frequency of flashes is not reproduced by phones, but the pulse train contains a component with audio frequencies, which are audible. Diode VD1 serves to close the extra current of phones at the end of the pulse and close the transistor VT3, it cuts off voltage surges, improving the quality and slightly increasing the volume of sound reproduction. The receiver is powered by a galvanic cell with a voltage of 1,5 V or a disk battery with a voltage of 1,2 V. The current consumption does not exceed 3 mA, if necessary, it can be set by selecting the resistor R4. Setting up the receiver begins with checking for generation by turning the knob of the variable resistor R2. It is detected by the appearance of a rather strong noise in telephones, or by observing a "saw" on the oscilloscope screen in the form of a voltage on capacitor C4. The superization frequency is selected by changing its capacitance, it also depends on the position of the variable resistor R2 slider. The closeness of the superization frequency to the frequency of the stereo subcarrier of 31,25 kHz or its second harmonic of 62,5 kHz should be avoided, otherwise beats that interfere with reception may be heard. Next, you need to set the tuning range of the receiver by changing the size of the loop antenna - an increase in diameter reduces the tuning frequency. You can increase the frequency not only by reducing the diameter of the frame itself, but also by increasing the diameter of the wire from which it is made. A good solution is to use a braided piece of coaxial cable coiled into a ring. The inductance also decreases when a frame is made from a copper tape or from two or three parallel wires with a diameter of 1,5-2 mm. The tuning range is quite wide, and the operation of its installation is not difficult to perform without instruments, focusing on the stations being listened to. In the VHF-2 (upper) range, the KT361 transistor sometimes works unstably - then it is replaced with a higher frequency one, for example, KT363. The disadvantage of the receiver is the noticeable effect of hands brought to the antenna on the tuning frequency. However, it is also characteristic of other receivers in which the antenna is connected directly to the oscillatory circuit. This drawback is eliminated by using an RF amplifier, as if "isolating" the superregenerator circuit from the antenna. Another useful purpose of such an amplifier is to eliminate the radiation of flashes of oscillations from the antenna, which almost completely eliminates interference to neighboring receivers. The RF gain should be very small, because both the gain and the sensitivity of the super-regenerator are quite high. These requirements are best met by a transistor URF according to a common-base or common-gate circuit. Turning again to foreign developments, we mention the super-regenerator circuit with URF on field-effect transistors [4]. Economical Super Regenerative Receiver In order to achieve extreme efficiency, the author developed a super-regenerative radio receiver (Fig. 6), which consumes less than 0,5 mA from a 3 V battery, and if the URF is abandoned, the current drops to 0,16 mA. At the same time, the sensitivity is about 1 μV. The signal from the antenna is fed to the emitter of the URF transistor VT1, connected according to the common base circuit. Since its input impedance is low, and taking into account the resistance of the resistor R1, we obtain an input impedance of the receiver of about 75 ohms, which allows the use of external antennas with reduction from a coaxial cable or VHF ribbon cable with a 300/75 ohm ferrite transformer. Such a need may arise at a distance of more than 100 km from radio stations. Capacitor C1 of small capacity serves as an elementary HPF, attenuating KB interference. In the best reception conditions, any surrogate wire antenna is suitable. The RF transistor operates at a collector voltage equal to the base voltage - about 0,5 V. This stabilizes the mode and eliminates the need for adjustment. The collector circuit includes a coupling coil L1 wound on the same frame with a loop coil L2. The coils contain 3 turns of PELSHO 0,25 wire and 5,75 turns of PEL 0,6, respectively. The frame diameter is 5,5 mm, the distance between the coils is 2 mm. The tap to the common wire is made from the 2nd turn of the L2 coil, counting from the output connected to the base of the VT2 transistor. To facilitate tuning, it is useful to equip the frame with a trimmer with an M4 thread made of magnetodielectric or brass. Another option that makes tuning easier is to replace capacitor C3 with a trimmer, with a capacitance change from 6 to 25 or from 8 to 30 pF. Tuning capacitor C4 type KPV, it contains one rotor and two stator plates. The super-regenerative cascade is assembled according to the already described scheme (see Fig. 1) on the transistor VT2. The operating mode is selected with a tuning resistor R4, the flash frequency (superization) depends on the capacitance of the capacitor C5. At the output of the cascade, a two-link low-pass filter R6C6R7C7 is turned on, which attenuates oscillations with a superization frequency at the input of the ultrasonic frequency converter so that the latter is not overloaded by them.
The super-regenerative stage used gives a small detected voltage and, as practice has shown, requires two AF voltage amplification stages. In the same receiver, the UZCH transistors operate in the microcurrent mode (pay attention to the high resistances of the load resistors), their amplification is less, so three voltage amplification stages (VT3-VT5 transistors) are used with a direct connection between them. The cascades are covered by the OOS through resistors R12, R13, which stabilizes their mode. For alternating current, the OOS is weakened by capacitor C9. Resistor R14 allows you to adjust the gain of the cascades within certain limits. The output stage is assembled according to the scheme of a push-pull emitter follower on complementary germanium transistors VT6, VT7. They operate without bias, but there are no step-type distortions, firstly, due to the low threshold voltage of germanium semiconductor devices (0,15 V instead of 0,5 V for silicon ones), and secondly, due to the fact that that oscillations with a superization frequency still penetrate a little through the low-pass filter into the ultrasonic frequency and, as it were, “blur” the step, acting like HF bias in tape recorders. Achieving high receiver efficiency requires the use of high-impedance headphones with a resistance of at least 1 kOhm. If the task of obtaining marginal efficiency is not set, it is advisable to use a more powerful final ultrasonic frequency converter. Establishing the receiver begins with UZCH. By selecting resistor R13, the voltage at the bases of transistors VT6, VT7 is set equal to half the supply voltage (1,5 V). They are convinced that there is no self-excitation at any position of the resistor R14 slider (preferably using an oscilloscope). It is useful to apply any sound signal with an amplitude of no more than a few millivolts to the input of the ultrasonic frequency converter and make sure that there are no distortions and the symmetry of the limitation during overload. By connecting a super-regenerative cascade, by adjusting the resistor R4, noise appears in the phones (the amplitude of the noise voltage at the output is about 0,3 V). It is useful to say that, in addition to those indicated in the diagram, any other silicon high-frequency transistors of the p-n-p structure work well in the URF and super-regenerative cascade. Now you can already try to receive radio stations by connecting the antenna to the circuit through a coupling capacitor with a capacity of not more than 1 pF or using a coupling coil. Next, the URF is connected and the range of received frequencies is adjusted by changing the inductance of the coil L2 and the capacitance of the capacitor C3. In conclusion, it should be noted that such a receiver, due to its high efficiency and sensitivity, can be used both in intercom systems and in burglar alarm devices. Unfortunately, the FM reception on the superregenerator is not the most optimal way: operation on the slope of the resonant curve already guarantees a deterioration in the signal-to-noise ratio by 6 dB. The non-linear mode of the super-regenerator is also not very conducive to high-quality reception, nevertheless, the sound quality turned out to be quite good. Literature
Author: V.Polyakov, Moscow
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