FM radio circuit design using super regenerative circuit

Examples of using super-regenerative radios to receive commercial FM radio stations can be found on major websites on the Internet. Because the circuit is very simple, it will be very impressive to laboratory visitors, especially children. However, the basic single-tube circuit shown in Figure 1 has many disadvantages. This design example introduces a better circuit.

The super regenerative circuit is basically an AM radio. It can demodulate wideband FM by changing FM to AM using one side of the tuning curve. Although this is a primitive way to demodulate FM signals, it works very well.

Figure 1: Self-quiescent single-tube super-regenerative FM radio.

A super regenerative tuner is a super regenerative circuit that goes in and out of oscillation at an ultrasonic rate (e.g. 25kHz). The rate at which the oscillation starts and stops is called the quench frequency. This frequency should be greater than the upper limit of human hearing, but should be as low as possible because a high quench frequency reduces the sensitivity of the receiver. The output to the RC integrator is a series of pulse width modulated quench frequency pulses. Finally, the integrator converts this pulse width modulated signal into an audio output.

The circuit shown in Figure 1 is a self-quiescent circuit. It is difficult to make this circuit quiescent. It is necessary to select the transistor with the highest gain and the appropriate operating voltage to obtain the best performance. In a self-quiescent circuit, the output may be distorted. In addition, the quiescent frequency may vary with tuning and may be too high or too low.

These problems can be solved by adding an extra oscillator to force the circuit to quiesce. This quiesced oscillator uses a two-transistor astable multivibrator. This results in a three-transistor circuit - still a fairly simple circuit for FM radio (Figure 2). Adjustment of the quiesce level control is critical here, so a good single-turn potentiometer should be used in this position. The quiesce frequency of this oscillator is about 21kHz.

Figure 2: Tuner with forced quiescence.

The antenna coil is made by air-winding 5 turns of AWG 18 wire on a 1/4" bobbin. The spacing between the windings is the width of one wire, so you can wind two windings closely on the bobbin first and then remove one set. After winding, you can remove the coil from the bobbin. Take out the tap at half a turn on the 12V power supply side to connect the antenna. A one-meter long wire antenna is enough, and then use a capacitor of 0.04pF to 6pF to tune the station.

The tuner also requires an audio amplifier. The output level of the tuner is below line level, so an amplifier with a microphone input is very useful. A musical instrument amplifier works well because it has a high-gain guitar input. Another solution is to use the design idea of ​​using three discrete transistors to build an op amp published in the December 2011 issue of Electronic Design Magazine (EDN) US. This gain-of-10 amplifier can boost the output signal to audio line level.

I listen to NPR FM, which is mostly talk. This station has two digital supplemental channels: a jazz/classical channel, and a main HD channel, both on the 90.7 MHz frequency. These supplemental programs and the stereo pilot signal do not interfere with the main analog signal received by the FM tuner described in this article.