Superheterodyne receiver with one transistor
This simplest ever circuit of the reflex superheterodyne radio receiver includes only two semiconductor components - a transistor and a diode. It can receive signals of MW or LW band. The intermediate frequency (IF) is 455 KHz.
The circuit (see Fig. 1.) uses standard components - the loopstick antenna (L1, L3), the heterodyne RF transformer (L2, L4) and the IF transformer (L5, L6) are from an AM radio. The receiver has very good selectivity, but the sensitivity is the same as for a good one-transistor receiver.
Fig. 1. Circuit diagram of the superheterodyne reflex receiver
T1 - 2N168A, D1 - 1N34 (germanium);
L1, L3 - a loopstick antenna from an AM radio;
L2, L4 - a heterodyne RF transformer;
L5, L6, C8 - a resonant RF transformer;
R1 - 250 Ω; R2 - a potentiometer of 5K; R3 - 270K;
C5, C8 - 10 nF; C6 - 0.05 μF; C7 - 10 μF x 6 V;
Headphones with high impedance;
Power supply - 3..4.5 V.
How this superheterodyne reflex receiver works. The loopstick antenna (L1, L3) receives the signal of a frequency that depends on the resonant tank L1C1C3. From the coupling coil L3 the signal goes to the base of the transistor T1. The transistor works as a heterodyne and as a mixer in the same time. The heterodyne frequency is determined by the resonant tank L2C2C4. If the resonant tank L1C1C3 of the loopstick antenna is tuned to 1 MHz, and the heterodyne is tuned to 1.455 MHz, the mix of this signals will produce IF frequency of 455 kHz (1.455 - 1.0 = 0.455 MHz), and the image frequency of 2.455 MHz (1.455 + 1.0 = 2.455 MHz). The last one will be rejected by the IF resonant tank L5C8.
The IF signal of 455 kHz goes through the coupling coil L6 to the diode D1. The diode detects the IF signal, and the audio signal develops across the potentiometer R2 (the volume control). The capacitor C8 blocks RF pulsations. By the way, this capacitor connects coils L3, L4 to the ground for AC current.
From the capacitor C8 the audio signal goes to the base of transistor T1. Coils L3, L4, L5 have very small impedance for audio frequencies, so do not affect the audio signal. From the collector of T1 the audio signal goes through coils L5, L2 to the headphones. To prevent any unnecessary RF feedback, the headphones are blocked by the capacitor C5. The same capacitor also provides connection for AC in the resonant tank L4C2C4.
The circuit diagram of this superheterodyne reflex radio was published in the magazine «Radio-Electronics» in December of 1960 year.