Fm Radio Circuit: The Logic Behind The Magic

If anyone asks the cause behind the advent of frequency modulation (read FM radio), let it suffice to say that it was an initiative taken to solve the problems regarding noise and fidelity that existed in AM broadcasting band. While the first FM radio circuit was much more complex than what we get to see today, comprising a super-heterodyne converter, a limiter stage, a wideband IF and a discriminator, the introduction of commercial FM broadcasting gave rise to comparatively simple FM radio circuit designs.

Making an FM radio circuit requires an understanding of the effects of coupling a FM signal to a tuned circuit with a high Q factor. If the FM signal deviates from the FM radio circuit's resonant frequency, the reception shall grow weaker; the moment it comes near the resonant frequency, the reception shall get stronger. The tuned circuit thus imposes amplitude changes on FM radio signals as the frequency alters. Crystal diodes are thus vastly used in FM radio circuits; the diodes are highly sensitive and can detect even the minutest amplitude changes, which helps in accurate conversions of amplitudes into signals. This is the secret behind the clarity and the fidelity delivered by the standard broadcast FM. The process is called slope detection, slope being the attenuation curve of the tuned FM circuit.

Sometimes, an oscillator that detects FM with similar efficiency replaces a crystal diode because of their non-linearity that detects the amplitude variations and the additive effect brought out by matching exactly the FM signal and the signal generated by the oscillator. The additive effect is the slightly stronger oscillator signal amplitude that diminishes with the distance of the FM signal from the perfectly matching frequency and vice versa.

Controlling the gain in an amplifier is a vital point in FM radio circuit designs. Here, it is built in a way that allows the operator to control. However, a balance must be maintained and sufficient positive feedback must be allowed, but not so much as to make it oscillate by setting it too high. A regenerative amplifier circuit, as it is called, is a high Q-tuned circuit that is inexpensive, can be assembled easily and functions satisfactorily.

When it comes to converting frequency to voltages, we are looking into simpler FM radio circuit designs. The voltage from the limiter (nonlinear electronic circuit whose output is limited in amplitude; used to limit the instantaneous amplitude of a waveform) transforms to uniform pulses that are delivered to a capacitor to charge it, while a resistor (connected parallel to the capacitor) discharges the capacitor voltage continually. If the pulse rate exceeds a critical frequency, the capacitor voltage (directly proportional to the pulses) increases and vice versa. But when it comes to more complex FM radio circuits, it is phased FM detection that is favored the most; in this case, the FM signal is split into two by the quadrature detector bringing forth a 90 degree phase difference between the split signals which then enter a balanced modulator. The FM signals are then mixed and the phase shift produces an amplitude variation, directly proportional to the frequency of the modulation.