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Note: this is not designed to be realistic, but rather to show roughly how it might be done. Also, this design is vulnerable to noise in the form of variance in the FM signal amplitude, so some stage before this circuit should fix the amplitude to something constant regardless of what comes through the antenna.
The blue signal is the FM. It varies between 99MHz and 101MHz with a period of 1MHz. The green signal is the blue signal mixed with a 90MHz source to produce a combined signal of 110MHz, 90MHz (leaked through the parasitic capacitance of the MOSFET), 10MHz, and a small amount of distortion. The orange signal is the green signal filtered to contain only the 10MHz component. The frequency of this component is the difference between the FM frequency and 90MHz, so it varies in frequency along with the FM signal. As such, it varies between 9MHz and 11MHz with a period of 1MHz. This process of decreasing the signal frequency with the use of another high-frequency source close in frequency to the wanted signal is called heterodyning, and the use of this process makes this FM receiver a superheterodyne FM receiver. Additionally, since I filtered the mixed signal with a low-pass filter, it’s stronger when at 9MHz and weaker when at 11MHz. As such, in addition to being an FM signal, the low-pass filter has added an AM component. Not all superheterodyne receivers use a low-pass filter in this way, but mine does and it seems to work in EC, and other demodulation methods such as use of a PLL (phase locked loop) may not be as easy to implement in EC.
Ok, so far the FM signal has been converted to a lower frequency and the FM modulation converted to AM modulation (although the FM component hasn’t been removed). The result of this is in the orange signal. Everything past the orange wire is an AM demodulator. The two diodes cut off the lower half of the signal so that the variance in amplitude turns to variance in average voltage, or in other words turns the 1MHz AM modulation into an actual 1MHz component. A low-pass filter then removes the 10MHz AC component leaving just the 1MHz component. As such, the 1MHz signal that the 100MHz signal was modulated with has been recovered. The result of this is the red signal. If this 1MHz signal was instead an audio signal or some such, it would be recovered just the same. Perhaps slightly distorted, but still pretty good.
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