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Super-regenerative receivers are characterized by high sensitivity and high gain with exceptional simplicity of circuit and design. Radio amateurs usually design self-extinguishing super-regenerators, sometimes capricious in tuning. Super-regenerators with an external source of damping oscillations are distinguished by the best parameters and stability in operation. It is this design that is proposed in the published article.

It is known that the sensitivity of super-regenerative receivers is limited by the inherent noise of the regenerative stage [1], which is largely determined by the noise properties of the used transistor. Despite the fact that field-effect transistors are less noisy than bipolar ones, in the literature there are practically no circuits of super-regenerators based on field-effect transistors. The variant of just such a receiver is offered to the attention of radio amateurs. Its essential advantages are high sensitivity (0,5 μV at a modulation depth of 0,9 and a signal-to-noise ratio of 12 dB), low current consumption (1,4 mA at a supply voltage of 4 V), a wide range of supply voltages (3... 9 V), low parasitic radiation (the superregenerator itself consumes a current of 80 μA).

External superization greatly simplifies receiver tuning and increases the stability of its operation. The receiver can be successfully used in the areas of application traditional for a superregenerator (in radio control equipment, the simplest radio stations, radio security devices, etc.).

The circuit diagram of the receiver is shown in fig. one.

The superregenerative detector is assembled on a low-noise transistor VT1. The cascade is an oscillator with autotransformer feedback. The generation frequency is determined by the parameters of the oscillatory circuit L1C2, tuned to 27,12 MHz. The use of a two-gate transistor greatly simplifies the implementation of the external superization mode. It is known that the slope of the characteristic for the first gate depends on the voltage at the second gate. When this voltage is zero, the slope is less than critical and there is no generation. A superization voltage with a frequency of 3 ... 60 kHz is supplied to the second gate through the potentiometer R70 from a generator assembled on elements DD1.1 and DD1.2. Capacitor C5 connects the second gate to a common wire at a high frequency and, in addition, gives the superization pulses a shape close to triangular. Adjusting the amplitude of superization pulses using potentiometer R3 allows you to smoothly change the time during which the steepness exceeds the critical value, and hence the duration of high-frequency flashes in the L1C2 circuit. Thus, it is possible to change the operating mode of the super-regenerator, setting either linear, at which maximum sensitivity is achieved, or nonlinear, at which AGC is most effectively implemented.

The load of the super-regenerative detector is the R6C6 low-pass filter. A useful signal with an amplitude of the order of 1 ... 3 mV from this filter is fed through the capacitor C9 to the ULF, which is used as the two remaining elements of the DD1 microcircuit. Negative DC feedback through the elements R5, R7, C10 ensures the operation of the digital microcircuit in a linear mode [2]. Elements C12, C13, R8 set the cutoff frequency of the frequency response of the amplifier to about 3 kHz.

Resistor R1 serves to form a negative (with respect to the source) bias voltage at the first gate, which ensures the initial value of the slope of the transistor VT1 is less than critical. The second function of this resistor is very significant. Its resistance determines the initial value of the DC component of the current through the transistor, and hence the level of intrinsic noise. With the values ​​of the elements indicated in the diagram, this current is only 80 ... 90 μA, which, among other things, makes the parasitic radiation of the superregenerator very small, since all the power it consumes from the power source does not exceed 0,5 mW.

Capacitor C3 is chosen to have a large capacitance, since it must shunt resistor R1 both at the carrier frequency and at the superization and envelope frequencies of the received signal.

The main characteristics of the receiver are shown in tables 1 and 2.

Construction and details. The printed circuit board of the receiver is shown in fig. 2 and has no special features.

With a slight deterioration in the characteristics of the receiver as VT1, domestic transistors of the KP306, KP350 series can be used, taking measures to protect them from static electricity during installation. It should be borne in mind that the KP327 series transistors are produced with a very large percentage of defects, but serviceable ones can be used. Capacitor C3 must be ceramic. It is permissible to replace it with any capacity not less than that indicated in the diagram, provided that a 1000 pF ceramic capacitor is connected in parallel. To ensure a stable superization frequency, capacitor C8 must be with a small TKE. The rest of the details can be of any type. The contour coil is wound on a frame with a diameter of 5 mm and contains 9 turns of wire with a diameter of 0,35-0,5 mm. The tap is made from the third from the bottom according to the coil scheme. A carbonyl iron core is screwed into the frame.

Since the load capacity of the K561LE5A chip is small, the device connected to the receiver output must have an input impedance of at least 30 kOhm. As a low-frequency amplifier, instead of elements DD1.3, DD1.4, you can use ULF of any design with a gain of at least 1000. At supply voltages of more than 5 V, for example, the economical op-amp K140UD1208 gives good results. The total current consumption at a supply voltage of 9 V does not exceed 1,5 mA. The multivibrator of auxiliary oscillations can also be assembled on transistors according to any known scheme. It is only important to maintain the required frequency and shape of the quenching pulses.

Receiver setup begins with checking the installation. Then you should set the slider of the variable resistor R3 to the left position according to the diagram, turn on the power (the nominal voltage is 4 V) and make sure that the constant voltage across the resistor R1 lies within 0,6 ... 0,7 V. Otherwise, the transistor is faulty and it needs to be replaced. By connecting the oscilloscope to pin 10 DD1.2, check the presence of rectangular pulses with a frequency of 60 ... 70 kHz. If necessary, specify the frequency by selecting the resistance of the resistor R4. By switching the oscilloscope to the receiver output and smoothly turning the potentiometer R3, low-frequency noise appears on the screen.

Now you can connect a standard signal generator to the antenna input, setting oscillations at its output with a frequency of 27,12 MHz, an amplitude of 100 μV and a modulation depth of 0,9. By rotating the core of the coil, the circuit is tuned to resonance at the maximum amplitude on the oscilloscope screen. Having returned the slider of the potentiometer R3 to its original position (the fluctuations at the receiver output will disappear), these oscillations should be restored by smooth rotation of the slider and its position should be found at which the voltage amplitude at the receiver output will stop increasing.

By reducing the input voltage to 1 μV (if necessary, refining the circuit setting), they control the correct position of the variable resistor slider. This setting corresponds to the non-linear mode of the super-regenerator.

A further increase in the superization voltage using R3 is impractical, since the useful signal increases slightly, while the noise increases significantly.

If now the R3 slider is turned in the opposite direction, a linear mode will be established, in which the signal-to-noise ratio improves slightly, but the output signal amplitude drops. It should be borne in mind that although the supply voltage interval at which the main parameters of the receiver are saved is 3 - 9 V, for each specifically selected voltage, it is necessary to clarify the optimal position of the variable resistor R3 slider using the above method.

In the absence of a GSS, you can use the transmitter with which the receiver is supposed to work, placing it at such a distance from the receiver that the output signal is not yet limited.

In conclusion, it should be noted that, like any superregenerator, the noise immunity of the receiver and its selectivity are low, since the bandwidth, which is numerically equal to several superization frequencies [1], is 120...140 kHz.

Literature

1. Belkin M.K. et al. Superregenerators. - M.: Radio and communication, 1983.
2. Frolov V. Simple direct amplification receivers. Radio Yearbook, 1985. - M.: DOSAAF.

Author: V.Dnishchenko, Samara

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