Encyclopedia of radio electronics and electrical engineering / radio reception
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This miniature radio receiver is assembled according to a reflex circuit and is used to receive radio stations in the long or medium wave range.
The radio signal, to the frequency of which the oscillatory circuit of the magnetic antenna WA1, consisting of the coil L1 and the capacitor C1, is tuned, is fed from the coupling coil L2 to the base of the transistor VT1 relative to its emitter.
Due to the presence of capacitor C2, the high-frequency transistor is connected according to a common emitter circuit, and resistor R1 is connected to the collector through capacitor C3. From the collector load R1, for additional amplification, the signal is directly fed to the base of the transistor VT2, also connected according to the common emitter circuit. The radio signal, amplified in two stages, from the resistor R2 is fed for detection to the diode VD1, from which the low-frequency component through the coil L2 again enters the base VT1 for low-frequency amplification. Now the capacitor C2 serves to filter the high-frequency component of the detected voltage, and the low-frequency transistor VT1 is connected according to the common collector circuit.
After amplification by the transistor VT2, the signal is fed to the output amplifier, assembled on the transistor VT3, which is loaded with the headphone BF1. Capacitor C3 blocks the power supply and the phone from flowing the high-frequency component of the current of the first stages.
A printed circuit board made of single-sided foil fiberglass and the arrangement of circuit elements on it are shown in the figure.
The L1 loop coil for the medium wave range is wound turn to turn on a paper sleeve, put on a 600NN brand ferrite rod with a diameter of 8 and a length of 77 mm, and contains 90 turns of PEL wire with a diameter of 0,2 mm.
The communication coil L2 is wound over the loop coil and contains 10 turns of the same wire. For the long wave range, the L1 coil contains 300 turns of PEL wire with a diameter of 0,1 mm, and the coupling coil contains 30 turns. Tuning capacitor C1 is variable with an air dielectric of the KP-180 type, but a semi-variable ceramic capacitor with a capacity of 150 pF can also be used. The sound emitter BF1 is a small-sized headphone of the TM-1 or TM-4 type, which has a DC resistance of about 100 ohms.
The advantage of this receiver is that it is especially economical: when powered by a single element 316, the current consumed by the receiver does not exceed 1,5 mA.
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