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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Regeneration mode in a super-regenerative receiver. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / radio reception The classic super-regenerator with "auto-superization" (Fig. 1), which has been repeatedly published since the 60s [1...3], has well-established applications in automation and telemechanics, burglar alarms, and short-range radio intercoms. Sometimes it is used as a receiving device for speech and music programs with low sound reproduction quality. Such a receiving device is distinguished by a rather high sensitivity, simple circuitry, small weight and size, and ease of repetition. Therefore, radio amateurs use it in their designs.
However, sometimes there are difficulties in tuning such a receiver, and some practical experience is required to master the tuning of a super-regenerative detector. According to the author, this is due to the spread of characteristics and parameters of transistors, differences in specific circuits and ratings of elements, as well as differences in design, which determine the capacitance of mounting and parasitic connections. Unfortunately, in the application to the superregenerator, these difficulties are not sufficiently reflected in the amateur radio literature. Once, when setting up such a "obstinate" super-regenerator, the author obtained high-quality reception of broadcasting stations with frequency modulation. This effect has been seen before, but has not been of interest, and therefore the mechanism of administration has not been determined. But this time, such signs as the absence of super-regenerative noises, the dependence of the received signal level on the value of positive feedback and on the bias level at the base of the transistor and, accordingly, on the collector current, which was equal to 0,2 in the operating mode, were carefully analyzed ... 0,3 mA. This is 3...4 times less than in the normal operating mode of a superregenerative detector. Based on these features, it was possible to determine the mode of regeneration. The mechanism for receiving frequency modulation (FM) in such a receiver is to convert frequency modulation (FM) to amplitude modulation (AM) on one of the slopes of the resonant characteristic of the circuit, and detect AM by the emitter junction of the transistor. The presence of FM to AM conversion is confirmed by the presence of a “dip” in the signal level at the central tuning of the circuit and a greater signal volume when tuning on the upper slope of the resonant characteristic of the circuit (the upper slope is always steeper than the lower one, and hence the conversion coefficient is higher). To the surprise of the author, the sensitivity and selectivity of such a superregenerator turned out to be sufficient for fairly high-quality reception in the range of 100...108 MHz. The main disadvantages of such a receiver: - low selectivity, which is expressed in the presence of weak signals from powerful and closely spaced stations in transmission pauses, which can be eliminated by increasing the degree of regeneration; In addition, all regenerative receivers have a dependence of the generation threshold and prethreshold gain on the frequency tuning, as well as the dependence of all these parameters on the supply voltage. At the indicated operating frequencies, the contour tuning and the generation threshold are strongly dependent on the capacitances introduced by the surrounding objects. Therefore, shielding of the regenerative detector is required. With all this, the simplicity of the circuit and the settings of such a receiver make it possible, in my opinion, to find application in amateur radio practice, for example, for broadcasting reception in the form of an on-air radio station or with tuning into several stations, as well as for receiving television sound in the meter wavelength range . The schematic diagram of the regenerative detector is shown in Fig.2. It is a three-point capacitive oscillator used in the underexcited mode. R1 and RP1 form an adjustable bias voltage divider based on the transistor. The collector current and, accordingly, the gain of the transistor depends on the magnitude of the bias. This effect allows you to adjust the level of regeneration with virtually no change in positive feedback.
The supply voltage for this divider and the entire detector is stabilized by the Zener diode VD1. When powered by galvanic batteries or a high-quality stabilizer, it can be excluded. This reduces energy consumption, but increases the dependence of the operating mode on the supply voltage. The base of the transistor is locked to a common wire by an electrolytic capacitor C2. This provides a low level of low-frequency noise at the output of the detector. Parallel to it, capacitor C4 is connected, blocking the base at high frequency. Resistor R3 contains both HF and LF signals, and thus the presence of negative feedback for both LF and HF is determined. The presence of negative RF feedback stabilizes the regeneration so strongly that the well-known hysteresis of the generation threshold in regenerators becomes practically undetectable. Therefore, the generation threshold when adjusting RP1 retains its position both during the forward and reverse strokes of the adjustment knob. Trimmer capacitor C6 provides positive feedback, the value of which is set during the initial setting. R4, C7 form a low-pass filter that highlights the audio signal. In this case, the filter cutoff frequency is 100 kHz, which allows you to connect a stereo decoder for stereo reception, as, for example, in [4]. The input circuit C5, L1 is connected to the antenna WA1 by inductive coupling using the coil L2. Inductive coupling allows eliminating interference from the AC mains to the antenna, as well as eliminating the effect of changing the circuit setting and the regenerator mode due to the capacitances introduced into the circuit from objects surrounding the antenna. The limits of capacitance change C5 are not critical, and any trimmer capacitor can be used instead. The antenna is a piece of mounting wire 0.5 ... 1 m long. The audio frequency amplifier can be assembled according to any scheme, as long as it provides sufficient reception volume. This scheme of the regenerative detector was tested on a mock-up assembled by surface mounting on a fiberglass plate using reference points, according to the Zhutyaev method [5]. Installation is not critical. However, novice radio amateurs, when repeating the circuit, should pay attention to the circuits associated with the emitter and collector of the transistor. The installation of these circuits must be very compact and the leads of the elements must be as short as possible. The same requirements apply to the circuit of the upper (according to the scheme) part of the oscillatory circuit. Capacitor C1 must be connected between the circuit and the common wire with links of a minimum length. If the regenerative detector is to be used for reception and not for experimentation, it should be placed in a screen. Capacitors C1, C4, C7 are necessarily ceramic. Their capacities are not critical. C2, C3 - electrolytic, any type. Transistor VT1 can also be replaced with another one, but with a limiting amplification frequency that is at least twice as high as the operating frequency. You can use pnp type transistors by changing the polarity of the power supply and electrolytic capacitors, and, in addition to silicon, germanium transistors can be used. For the frequency range 100 ... 108 MHz, the coil L1 is a half-turn with a diameter of 30 mm with a linear part of 20 mm. Wire - 1 mm in diameter. L2 at the same time has 2 ... 3 turns with a diameter of 15 mm from a wire with a diameter of 0,7 mm, located inside the half-turn. For the range 66 ... 73 MHz, L1 has 5 turns with a diameter of 5 mm from a wire with a diameter of 0,7 mm in increments of 1 ... 2 mm. L2 at the same time has 2 ... 3 turns of the same diameter from the same wire. The coils are frameless and are arranged parallel to each other. Adjustment of the regenerative detector consists in setting the limits for bias adjustment based on the transistor by selecting R1. The collector current must not exceed 0,5 mA. In addition, capacitor C6 establishes a positive feedback of such a magnitude that, at the middle positions of the tuning and regeneration adjustment knobs, the generation threshold is reached. This shows up as a dull click followed by noise and possibly AC hum. And the last is the tuning of the circuit to the required frequency range. Such a receiver can operate in areas with a sufficiently high signal level. These are mainly large cities and areas around them. To increase the sensitivity, a single or two-stage high-frequency amplifier can be used. In this case, possible radiation to the antenna will be eliminated. The conducted studies of the circuit suggest the possibility of using such a receiver to receive television sound in the decimeter range. Literature 1. Transistor receiver for radio controlled models. - Radio, 1963, No. 10, S. 60. Author: E. Solodovnikov, Krasnodar; Publication: N. Bolshakov, rf.atnn.ru
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