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I know the theory of FM, but I just can't understand how this simple circuit works. So let's say a radio station is transmitted on 100 MHz, as I understand in this case the oscillator should also produce a 100 MHz sign. What I don't get is, how the FM bandwidth (which is around 100 MHz in a range) is picked up from the antenna and demodulated?

Could you please explain how this circuit works?

Thanks!

enter image description here

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    \$\begingroup\$ Where does this schematic come from? Do they say anything there about how it works? My guess is that it could work because this circuit is an oscillator which can be influenced (pulled) by the received signal. Just as changing the DC biasing of the oscillator changes its frequency, a forced frequency change might influence its biasing and that change is the demodulated signal. My guess is also that this circuit is extremely insensitive and will also disturb all FM receivers in the neighborhood. \$\endgroup\$ Feb 2, 2020 at 11:54
  • \$\begingroup\$ Original source seems to be: "The Simplest FM Receiver" from the book "Radio Receivers, from crystal set to stereo" \$\endgroup\$
    – SamGibson
    Feb 2, 2020 at 12:13
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    \$\begingroup\$ Just a nit: " the FM bandwidth (which is around 100 MHz in a range)" -- nope. The bandwidth of FM is something like 100kHz -- that 100kHz is centered around a carrier that's from 88 to 108MHz, but the bandwidth is still much narrow than the carrier frequency. \$\endgroup\$
    – TimWescott
    Feb 2, 2020 at 14:55

1 Answer 1

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The inductor L, the capacitor C and multiple (intrinsic) capacitances of the transistors form an parallel LC-tank circuit with resonance frequency of about 100MHz. The trim-capacitor C allows tuning this LC-tank resonance frequency.

Such a parallel LC-tank has its highest impedance at resonance frequency, thus it also has its highest voltage at resonance frequency. Every frequency deviation from resonance frequency leads to an amplitude change of the voltage at the left terminal of C1. This voltage is then coupled through C1 to the earphones.

For the FM-demodulation to work, the LC-tank has to be tuned a bit off resonance, so that the radio works on the slope of the resonance curve and thus the change of frequency results in a (mostly) linear change of voltage (this technique is called slope demodulation).

So in the end, this LC-tank acts like a (delta-) frequency to (delta-) voltage converter which is the basic function of an FM-receiver.

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  • \$\begingroup\$ You left out the part where to get it to work you have to tune it a bit off resonance, so that the radio works on the slope of the resonance curve (and, in fact, this technique is called slope demodulation). \$\endgroup\$
    – TimWescott
    Feb 2, 2020 at 14:57
  • \$\begingroup\$ @TimWescott Thanks, I added your note to my answer. \$\endgroup\$ Feb 2, 2020 at 15:07

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