Design Idea; Forced quenching improves three-transistor FM tuner
(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
The antenna coil is composed of five turns of space wound AWG 18 wire on a 1/4-in. (6 mm) former. The windings are spaced one wire-width apart, so you can close-wind two windings on the former and then remove one of them. When formed, the coil can be removed from the former. The coil is tapped one half turn from the 12V supply side to make a connection for the antenna. A one metre long wire antenna is sufficient. The 0.04 to 6 pF capacitor is used to tune in stations.
The tuner needs an audio power amplifier. The output level of the tuner is below line level, so an amplifier with a microphone input would be useful. A musical instrument amplifier would work fine due to the high gain guitar input. Another solution is to use the circuit: Build an op amp with three discrete transistors in the Design Ideas section of the Dec. 1, 2011 issue of EDN magazine. This amplifier with a gain of ten will boost the output to audio line level.
I listen to an NPR FM [National Public Radio in the USA – ed.] station that is mostly talk. The station has two digital auxiliary channels: jazz/classical, and the main channel in HD, all on the same frequency of 90.7 MHz. These extra programs and the stereo pilot do not interfere with the main analogue signal as received on the FM tuner described here.
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