Ganged Tuning

AM radios have a tunable LC circuit at their front end, whose resonant frequency can be adjusted to tune into different radio stations. The inductance is fixed, and comes mostly from a rod and coil inductor that functions as an antenna as well, although some stray inductance will also be present. If the fixed inductor is connected in parallel with a variable capacitor, the circuit’s resonant frequency, and therefore the radio station it selects, can be changed by rotating the capacitor spindle.

A traditional variable or tuning capacitor comprises two sets of metal plates - rotor and stator – arranged so that the rotor plates move between the stator plates as the capacitor’s spindle is rotated. The air between them acts as the dielectric. The component’s capacitance value varies as the rotor plates move between the stator plates, and change their degree of overlap.

Many designs call for multiple circuits to be tuned simultaneously. One example is a receiver circuit comprising a preselector, an input filter and corresponding oscillator. This requirement can be met by using a ganged tuning capacitor, in which several tuning sections are assembled onto a single shaft.

These metal plate designs are too large for today’s radios, so they have been largely replaced by smaller components such as miniature plastic tuning types. These are available with four or more ganged sections for use within AM/FM radios.