Encyclopedia of radio electronics and electrical engineering / radio reception
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The construction of simple radio receivers has been an old pastime of radio amateurs for many generations. It is especially interesting when the receiver circuit is somewhat unusual.
Here, a description is given of an experimental HF broadcasting receiver based on three transistors, the high-frequency part of which is built according to the direct amplification regenerator circuit. Despite the simplicity of the circuit with the receiver, you can "torment yourself to your heart's content" by setting it up. Well-tuned in terms of long-range reception ability, it can compete with more complex superheterodyne "brothers".
The receiver covers the range from 3,7 to 13 MHz, covering the ranges of 25, 31, 41, 49 and partially 75 meters. The antenna is a piece of mounting wire, with the length and position of which you can also experiment a lot and for a long time. Power supply - from a 9V battery (one "Krona" or two "flat" 4,5V each). Works - on the headphones from the player.
The circuit is shown in the figure.
(click to enlarge)
The high-frequency path is made on two transistors VT1 and VT2. If you remove the capacitor C3, then this is a completely ordinary two-stage URC, with the first stage on a field-effect transistor, which allows you to connect the input circuit directly, and the second stage on VT2, connected according to a common emitter circuit. There is also a germanium diode detector.
In this version (without C3), the receive path operates according to the direct amplification scheme. Amplifies the signals selected by the C1-L1 circuit and detects them with a diode detector on VD1. Such a scheme, especially on HF, has two important drawbacks - low sensitivity and very negligible selectivity in the adjacent channel.
With the capacitor C3 connected, a positive feedback occurs, which is regulated by a variable resistor R7 and a change in the capacitance of C3 itself. Resistor R7 can smoothly adjust the depth of the PIC from zero, when the circuit operates as a conventional direct amplification receiver, to the level when the URC turns into an RF generator that generates oscillations at the input circuit tuning frequency. Somewhere on the verge of the beginning of generation, you can set such a mode that the URF will generate as a generator driven by the input signal, and the oscillations generated by it will be modulated by AM according to the input signal. This line is very thin, but it allows, by fine tuning R7 (and C3), to achieve very high sensitivity and selectivity. So, turning the tuning knobs (C1) and resistors R7 and R8 (alternately, together, simultaneously ...) you can find a lot of interesting and unexpected things in the operation of this circuit. You can even manage to demodulate the SSB signals of amateur radio stations in the 40M band.
Coil L1 is frameless, pre-wound on a wooden stick with a diameter of about 30 mm. Contains 18 turns of PEV 1,2 wire. The withdrawal is made from the 5th turn. After winding, - remove from the stick, mold and cut the leads, strip and solder the outlet wire.
Author: A.Ivanov
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