FM demodulation with RLC circuits applet

Posted on Feb 7, 2014

Commercial FM demodulation is done at the IF frequency of 10. 7MHz. With a frequency deviation of ±75kHz, the deviation of the IF carrier is ±0. 7%. With this deviation, it is possible to convert the FM to AM or PM and AM using passive RLC circuits. The resulting AM is then demodulated with an envelope detector. Some basic FM demodulators are exam

FM demodulation with RLC circuits applet
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ined in the applet on this page. The possibility of converting a FM to an AM signal using first order RC and 2nd order Butterworth filters is first briefly examined. In the applet the Diode RC combination is assumed to be an ideal envelope detector. The practical envelope will be lower in amplitude and phase delayed compared to that shown in the applet. The demodulators used here are shown below. Although some are no longer used, understanding them makes understanding the more complex ones easier. Fn = 1 shows the FM passed through a simple 1st order system. Because of the variation of the response with frequency, then the signal an AM component which can be demodulated. In this case the output is small and distorted. Fn = 2 to 7 show special cases of Fn = 1. Image below show the case when the FM is centered at the frequency of maximum slope. Green shows an amplified version of the envelope. Fn = 11 is shown below. The signal at node 1 is shown in Yellow and its envelope in Green in the lower part of the image. The envelope is visibly distorted. Assuming the center frequency and BW of the FM are shown by the brown lines, then the resonant frequencies must be adjusted as shown below. The difference between the two responses is shown by the blue magenta curve. Both the resonant frequencies and Q`s should be adjusted so the blue magenta line is linear over the BW of the FM. The LS1-C1 response is shown by the green to blue...

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