Design Idea; Forced quenching improves three-transistor FM tuner

March 21, 2016 // By Graham Prophet
The idea of using a super-regenerative radio to receive commercial FM stations can be found at various locations on the Internet. Because the circuit is so small, it might be impressive to lab visitors, especially to children. But the basic one-transistor circuit shown in Figure 1 has drawbacks; a better circuit is the subject of this Design Idea.

(Editor’s note; because of the possibility of re-transmitted interfering signals, super-regenerative radios may require additional antenna filtering, or may not be permitted in some locations.)


The super-regenerative circuit is basically an AM radio. But wideband FM is demodulated by using one side of the tuning curve to change FM to AM ["slope detection"]. While this is a crude way to demodulate FM, it nevertheless works quite well.



Figure 1 Self-quenched one-transistor super-regenerative FM tuner


The super-regenerative tuner is a regenerative circuit that is brought into and out of oscillation at an ultrasonic sonic rate – for instance, 25 kHz. The rate that the oscillation is switched off and on is called the quench frequency. This frequency should be above the limit of human hearing, but otherwise as low as possible. High quench frequencies reduce the sensitivity of the receiver. The output to the RC integrator circuit is a series of pulses at the quench frequency that are pulse-width-modulated. The integrator changes this pulse-width modulation to an audio output.


The circuit in Figure 1 is a self-quenched circuit. It can be difficult to get the circuit to quench and therefore to operate. It may be necessary to select transistors for the highest gain and to select the operating voltage for best performance. In the self-quenched circuit, the output may be distorted. Also, the quench frequency may vary with tuning and it may be too high or too low.


Adding an external oscillator to force the quenching of the circuit solves these problems. A two-transistor astable multivibrator is used for this quenching oscillator. This makes a three-transistor circuit – still quite simple for an FM radio (Figure 2). The adjustment of the quench-level control is critical, so a good single-turn potentiometer should be used in this position. The quench frequency of this oscillator is about 21 kHz.



Figure 2 The tuner with forced quenching