Differences Between F.M. and A.M. Receivers

Author: J.B. Hoag

Superheterodyne receivers are used in the reception of frequency-modulated signals, as in Fig. 33 C.

Fig. 33 C. Block diagram of an f.m. receiver

Although much like those used with amplitude modulation, they differ in several important points.

1. Since f.m. is used at the ultra-high frequencies (28 MHz or higher)

   where there is sufficient room for their wide bands, the intermediate frequency is usually chosen between 4 MHz and 5 MHz.

2. The r.f. and i.f. tuned circuits must respond equally over the comparatively wide band which is used (20 kHz to 40 kHz).

3. The i.f. stages must apply a large voltage to the " limiter," even with weak input signals.

4. A limiter is used, over and above the usual circuits of a superhet.

5. A special detector, called a " discriminator," is used. The a.f. stages are the same as for amplitude-modulation receivers.