Encyclopedia of radio electronics and electrical engineering / radio reception

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Q-multipliers (regenerative amplifiers) have become very popular among radio athletes. Unlike the well-known multiplier, made in the form of a prefix connected in parallel with the converter circuit ("Radio", 1962, No. 4), the design described below is a special receiver unit, moreover, it uses the method of "serial passage" of the signal and provides for a smooth transition from the mode of suppression ("cutting") of the interference signal to the mode of selection of the useful signal. With the right (according to the diagram) position of the R6 potentiometer slider, the signal path with an inverted phase is difficult due to the large attenuation in this potentiometer. Therefore, the degree of signal amplification is determined by the quality factor of the circuit, and the signal being extracted is fed through the R3C2 R10C9 circuit to the output of the multiplier.

When the R6 slider is moved to the left extreme position, the amplitude of the signal with an inverted phase prevails, and the multiplier will operate in the suppression mode. The role of the "suppressor" of the frequency spectrum is performed by a circuit, the phase of oscillations in which is opposite to the phase in the anode circuit of the left triode L1. in the intermediate positions of the R6 slider, various options for the predominance of amplitudes with different phases are possible, which makes it possible to change the suppression and selection frequencies.

Using the Q-multiplier, the circuit diagram of which is shown in the figure, it is possible to select or suppress a narrow section of frequencies in the passband of the IF amplifier at 465 or 1600 kHz. The multiplier is included in the break in the circuit of the control grid of the lamp of the first stage of the IF amplifier, and the receiver to which the multiplier is connected is not subjected to any alterations. It is necessary to turn on the multiplier in the specified place of the receiver to reduce the cross modulation that occurs in the IF amplifier, and also because the multiplier works normally only when the IF voltage at its input has a small amplitude.

The multiplier allows you to suppress the desired frequency spectrum almost completely (to the noise level). It has separate adjustments for the width of the suppressed (or selected) frequency spectrum and the degree of suppression (selection).

By operating the multiplier in the minimum spectrum extraction mode, the degree of selective fading can be reduced. In order to maintain a normal timbre of sound under these conditions, it is necessary to connect a single-stage high-pass filter with a cutoff frequency of 200-400 Hz to the input of the low-pass amplifier.

It should be noted that this Q-multiplier, like other narrow-band devices, works effectively only when a stable local oscillator is installed in the receiver. The frequency shift of the local oscillator to 300...2000 Hz dramatically changes the mode of operation of the multiplier, since its band is within the same limits.

The multiplier is assembled on a 6NZP lamp. The cascade on the left (according to the diagram) triode of this lamp is a phase inverter with divided loads, and on the right triode it is a regenerative amplifier with capacitive feedback, that is, the Q-multiplier itself.

The operation of the multiplier in the mode of selection or absorption of the frequency spectrum, as well as the degree of selection or absorption is determined by the position of the potentiometer R6 slider. When the R6 slider is in the left (according to the scheme) position, suppression occurs, and in the right position, the frequency spectrum is selected, the width of which can be changed using the R8 potentiometer. The multiplier is tuned to certain frequencies within the passband of the IF amplifier using a variable capacitor C5.

The multiplier is assembled in a separate case together with the P1 switch, which is used as a two-pole toggle switch in two directions. The wires connecting the multiplier to the toggle switch should be as short as possible and carefully shielded. Coil L1, capacitors C2 - C6, as well as resistors R3 and R10, are mounted on a separate getinax board 0,5 mm thick with dimensions 35x65 mm. The board is covered with a screen measuring 36x36x67 mm.

Coil L1 is located in a pot-shaped carbonyl iron core SB-1a. It is wound with a licensor 7x0,07 on a three-section core frame in equal parts in each section. In the multiplier for the IF amplifier at 465 kHz, the coil L1 contains 60 turns (3x20), and for the IF amplifier at 1600 kHz - 30 turns (3x10). As C5, a single-section variable capacitor is used, which is installed in direct amplification pocket receivers.

When mounting a multiplier, special attention should be paid to reducing the mounting capacitance between the anode and grid circuits of its lamp. Significant mounting capacitance leads to excitation of the multiplier slightly below its tuning frequency or significant passband ripple of the IF amplifier. In some cases, to eliminate this phenomenon, it may be useful to include resistors of 20 ... 100 ohms in the places indicated by crosses in the diagram. These resistors should be located in close proximity to the L1 coil.

The best way to set up a multiplier is with a sweep generator and an oscilloscope. If these devices are not available, you can adjust it by ear.

To do this, set the R8 potentiometer slider to a position in which the multiplier is at the excitation threshold, and the R6 potentiometer slider is in the rightmost position (according to the diagram). In this case, the multiplier will operate in the frequency spectrum selection mode. The receiver is tuned with a multiplier connected to it to some station and, by rotating the rotor of the variable capacitor C5 in the multiplier, a sharp decrease in the level of higher transmission frequencies is achieved. When this is achieved, the multiplier will be tuned to the carrier frequency of the received station.

After making sure that the multiplier is working correctly in the selection mode, move the potentiometer R6 slider to the left side (according to the diagram) until the multiplier goes into suppression mode, which can be recognized by the occurrence of strong non-linear distortions as a result of a decrease in the carrier frequency level. After that, the variable capacitor C5 is adjusted to the maximum level of non-linear distortion. By checking the operation of the multiplier in the suppression mode, its adjustment ends.

It is possible that in order to compensate for the detuning of the grid circuit of the first IF transformer, which may occur when the multiplier is turned on, it will be necessary to connect a tuning capacitor Sk with a capacity of 1 ... 4 pF to the switch P15a (indicated by a dotted line in the diagram).

In parentheses in the diagram are the nominal values ​​​​of some parts that should be installed in the manufacture of a multiplier for an IF amplifier at 1600 kHz. Mounting such a multiplier should be especially careful.

Author: A.Bachinsky

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