fm receiver basics


Posted on Feb 5, 2014

Mainly f. m. receivers are of the superhetrodyne variety. Before we go into any depth about f. m. radio receivers let`s consider the principal differences between a. m. and f. m. signals. At first glance it might seem I am merely stating the blinding obvious but the differences are indeed quite profound. An a. m. receiver relies upon the original carrier signal (station frequency)


fm receiver basics
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having been amplitude modulated. This means the original amplitude (strength) varies at an audio rate. Looking at figure 1 we can see an unmodulated carrier signal as it might be seen on an oscilloscope. as you can see the amplitude of the carrier signal is unvarying, it remains constant in height looking from the top of the figure to the bottom of the figure. This carrier is common to both a. m. and f. m. signals. Perhaps the a. m. carrier signal repeats each cycle from point (a) to point (b) - "blue" - in figure 2 below at the rate of 810, 000 times a second, this represents a frequency of 810 Khz and would be in the a. m. radio band. Here you will notice that the audio modulating signal which is depicted in red has varied the strength of the carrier signal which is depicted green for purposes of this illustration. You will note my skills as a graphic artist leave much to be desired (hint: anyone able to contribute oscillograghs in. jpg or. gif formats ) but you should be able to see the carrier sine wave envelope is being varied in strength by the red audio signal. In the receiver circuit a diode detector can convert that envelope above back into the original audio signal for later amplification although some distortion does result. It was to an extent this distortion property that people sought a better means of transmission. More important it was discovered that noise (either man made QRM or natural noise QRN) was...




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