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Smooth feedback control is the main condition for the good operation of a shortwave receiver. If in conventional broadcasting receivers feedback plays only an auxiliary role, improving their performance, then in shortwave receivers it is of decisive importance.

There are dozens of feedback adjustment schemes. They can be divided mainly into three categories: the first is the adjustment by means of a moving feedback coil, the second is the adjustment by a variable capacitor, and the third is the adjustment by a variable resistance.

Let us briefly consider the most common of these schemes and find out their main advantages and disadvantages.

On fig. 1 shows a diagram in which the feedback is adjusted using a movable feedback coil L0 In practice, the adjustment is made by smoothly approaching or moving it away from the loop coil Lk, i.e., by changing the value of the mutual inductance between them. This scheme, widespread in the early years of amateur radio and sometimes used today, must be considered of little use for shortwave receivers. Its main disadvantages are the bulkiness and complexity of the device for the smooth movement of the feedback coil and the strong influence of the position of this coil on the tuning of the circuit, as a result of which the tuning of the circuit changes when the feedback is adjusted. This prevents any accurate calibration of the receiver.

Fig. 1

On fig. Figures 2, 3, and 4 show more advanced capacitive feedback control circuits. Scheme fig. 2 is known as the Reinartz scheme, the scheme in Fig. 3 - Wigant circuits and the circuit of Fig. 4 - Schiell schemes. Despite the fact that the feedback control here is capacitive, in all these circuits there are separate feedback coils L0, but they are stationary, wound in most cases next to the circuit coil on the same frame. The feedback value is controlled by changing the capacitance of the variable feedback capacitor C0.

Fig. 2

Fig. 3

Fig. 4

For the efficient operation of these circuits, it is necessary to include a cascade of a high-frequency short-wave choke Dr in the anode circuit, which blocks the path of high-frequency currents. Capacitor C in these circuits is a safety capacitor in case of a short circuit between the plates of the feedback variable capacitor. The performance of these circuits is about the same. However, the Reinartz circuit has the significant drawback that, since the plates of the variable capacitor are not grounded in it, the approach of the hands to the feedback capacitor has a rather strong effect on the tuning of the receiver and on the magnitude of the feedback. Wigant and Shkell ​​circuits do not have this drawback, which makes it possible to place capacitor C0 directly on the front panel in receivers. Therefore, the last two schemes have become widespread among shortwavers.

Capacitive feedback control circuits are superior to moving coil control circuits. However, they also have certain disadvantages. Firstly, they require additional parts - a variable capacitor, a choke; secondly, and most importantly, they do not completely exclude the dependence of the receiver tuning on the feedback control, although this phenomenon affects to a much lesser extent than when adjusting the feedback with a moving coil.

Figures 5, 6 and 7 show feedback control circuits using variable resistance. Feedback in the circuit of fig. 5 is regulated by changing the anode voltage. This is achieved by changing the resistance value (high-resistance) R. Capacitor C is a shunt, it provides the passage of the high-frequency component of the anode current

Fig. 5

In the scheme of Fig. 6 high-resistance variable resistance replaces a special lamp. Changing the incandescence of the lamp using the filament rheostat R1 causes a change in the magnitude of the current flowing through it, as a result of which the voltage at the anode of the detector lamp changes. This method of adjusting the feedback is used, among other things, in the well-known factory receiver KUB-4.

Fig. 6

In the scheme of Fig. 7 feedback adjustment is carried out using a variable resistance R, 500-1000 K, connected in parallel to the feedback coil.

Fig. 7

The indicated schemes for adjusting feedback with variable resistances have not found significant distribution among radio amateurs, mainly due to the imperfection of the design of variable resistances. In addition, variable resistances create significant rustles and noises that make tuning difficult. The scheme of Fig. 6 is free from these shortcomings. XNUMX, but it is much more complicated, since it requires the use of an extra lamp.

The use of tetrodes and pentodes in the detector cascades made it possible to carry out a more perfect feedback control using a variable resistance included in the screening grid circuit.

Figure 8 shows the most advanced and widespread of the existing schemes, the so-called Dow scheme. In this scheme, the loop coil is the entire coil Lk. The part of this coil between its grounded end and the tap is the feedback coil L0. The amount of feedback is adjusted by changing the voltage on the screen grid of the lamp. In practice, this is done by changing the value of the variable resistance R. Capacitor C serves here, as well as in the circuits of Fig. 5 and 6. for the passage of high frequency currents. The Dow circuit requires the inclusion of a high-frequency short-wave choke Dr. in the anode circuit of the lamp. The use of low-capacity capacitors C1 and C2 usually improves the performance of the cascade.

On fig. 8 shows a diagram of a Dow with a heating lamp.

Fig. 8

On fig. 9 shows the same circuit with a battery lamp. In the latter case, as can be seen from the diagram, it is necessary to use a second high-frequency inductor Dr in the lamp filament circuit.

Fig. 9

The above schemes are far from limited to all possible ways to adjust the feedback. There are a lot of them, as already mentioned. Only the most characteristic are described here.

Dow circuits are among the best for simple shortwave receiver applications. They give very smooth and stable feedback control. On all short-wave subranges, the adjustment is not accompanied by noise and rustles. The effect of feedback adjustment on receiver tuning is negligible. These circuits can be recommended to all amateurs when using pentodes or shielded lamps in detector stages. In the case of using a triode at the detector site, one of the circuits shown in Fig. 3 and 4 (Wigant and Schnell schemes). Using them by beginner radio amateurs should give the best results. Obtaining sufficiently effective results from other schemes is available only to qualified radio amateurs.

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