Encyclopedia of radio electronics and electrical engineering / radio reception

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Perhaps this scheme would have been more appropriate in some magazine of the 70s. But there is one important point, in my opinion, - the output of this simplest AM receiver is not a speaker or a telephone, but a piezoelectric sound emitter, like ZP-1, only imported. Of course, the sound quality is very average, since this sound emitter has a very narrow and uneven frequency response, so listening to music is unlikely to be enjoyable, but you can listen to the news - it sounds strange, but legible.

The receiver is built according to a regenerative circuit, which allows, despite the small number of stages, to obtain relatively good selectivity.

A signal in the range of long or medium waves (depending on the number of turns of the L1 coil) is received by a magnetic antenna based on the L1 coil. The magnetic antenna is a ferrite rod with a diameter of 8 mm and a length of 80 mm, on which the coil is located. A more miniature version is quite possible, for example, using lamellar ferrite or without a magnetic antenna at all (just an input circuit), and reception will be carried out on a surrogate antenna made in the form of a piece of mounting wire connected to the circuit.

The radio station signal is selected by setting the input circuit using a variable capacitor CV1, and enters through the isolation capacitor C1 to the base of the transistor VT1. This transistor works as an RF amplifier and detector. The cascade has a sufficiently large input impedance so that the input circuit can be connected to it without the use of transformer or autotransformer matching (without coupling coils or taps to the VT1 base).

The collector junction of the transistor VT1 is also a detector. Capacitor C2 forms a simple filter that suppresses the high-frequency component. Next, the detected low-frequency signal is fed to the VT2 base, which is a low-frequency amplifier. A load resistor R3 is included in its collector circuit, in parallel with which a piezoelectric sound emitter is connected. Resistor R3 is necessary here, since the operation of the cascade requires flow in the collector circuit, that is, through the load, not only alternating, but also direct current. The piezoelectric sound emitter has a very high resistance (it is practically a capacitor) and the cascade simply cannot work without R3. And here, the alternating voltage falling on this resistor enters F1 and makes it sound.

Let's go back to UHF on VT1. To improve the selectivity and sensitivity of the receiver, an adjustable positive feedback from the VT1 emitter to the base through the input circuit is introduced in the UHF circuit. The depth of this connection depends on the number of turns of the loop coil tap and on the position of the variable resistor RV1. This resistor simultaneously regulates the current through the transistor and the POS value, moreover, as the current decreases, the POS increases. This achieves a smoother adjustment. In the highest position of the variable resistor in the circuit, the circuit can switch to the generation mode. The task, when receiving, is to choose such a position of RV1, in which the circuit operates almost at the very threshold of generation. At the same time, sensitivity and selectivity increase sharply, and such a simple receiver makes it possible to receive very remote radio stations, especially at night.

Variable capacitor - one section of a variable capacitor from a portable Chinese receiver with AM bands (the same is from a simple analog radio). In general, any variable capacitor with a minimum capacitance of not more than 12 pF, and a maximum capacitance of at least 200 is suitable. 1th, winding - turn to turn. For long waves - 75 turns of thinner wire (0,3-0,6 mm), with a branch from the 10th, wound in several sections with a width of not more than 240 mm - each section in bulk.

L1 should not be wound directly on the rod, but on a pre-glued thick paper sleeve (drawing paper is just what you need).

Transistors KT3102 can be replaced with KT315. It is advisable to use a transistor with the highest coefficient h1e in place of VT21.

The cascades are connected according to a circuit with galvanic coupling and a self-setting mode of operation, therefore, when choosing resistors of those nominal resistances that are signed on the circuit, no setting of the DC mode is required. In general, adjustment as such is not required; in practice, adjustment is performed by a variable resistor RV1 during the operation of the receiver. The receiver is very interesting for experiments on long-range reception on such a minimalist device. If an external antenna is connected to the input circuit, the number of radio stations that can be heard with confidence increases dramatically.

Author: A.Ivanov

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