Low Tech FM Radios

Posted on Feb 4, 2014

Broadcast band frequency modulation (FM) radio was invented to solve existing problems with noise and fidelity on the amplitude modulation (AM) broadcast band. Thus, the first FM receivers were quite complex in design, employing a superheterodyne converter, a wideband IF, a limiter stage, and a discriminator. Unlike the first AM radio sets, the ea

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rliest FM radio sets did not use the simplest possible methods for receiving signals. Perhaps Armstrong, the inventor of most modern radio methods, was fully aware of all the ways to receive FM. (Someone reading these comments might want to offer the technical history so I can add it here. ) But, it was not until much after the introduction of commercial broadcast FM that simple FM receiver designs were published or sold. Although the title alludes to simplicity, these radio designs are not uniformly simple. These designs generally have low component counts, however the design or construction my have been far from simple. Believe it or not, you can actually make an FM radio crystal set. The key to understanding how this simple circuit can decode FM is to understand what happens when an FM signal is coupled to a tuned circuit. A good tuned circuit (one with a high "Q factor") will attenuate signals that are not near its resonant frequency. If you apply a FM signal to a good tuned circuit, as the frequency of the FM signal moves away from the resonant frequency of the tuned circuit, the tuned circuit will attenuate that signal. As the frequency of the FM signal moves toward the resonant frequency of the tuned circuit, the tuned circuit will let more of the signal pass through. Thus, a tuned circuit will impose amplitude changes on a FM radio signal that match the frequency changes. A crystal diode is sensitive to amplitude...

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