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I saw this simple schematic for an FM receiver:
https://www.mikroe.com/ebooks/radio-receivers-from-crystal-set-to-stereo/the-simplest-fm-receiver
Moreover, I saw some videos online where people have built it successfully:
https://www.youtube.com/watch?v=h_F3J4vyzNk
Starting at 2:58 on the video timeline.

I'm very puzzled as to how this circuit works. It's not mentioned in any electronic comm. texts, even as it's surprisingly simple for a circuit that successfully performs FM reception, filtering and demodulation.

I understand there is a Collpits oscillator at the heart of it that's tuned to the frequency of the station you want to filter, but that's where my understanding ends.

How does it turn frequency into voltage across the load resistor (R1) (schematic in the first link above) ?

I don't mind about the debiasing and audio amplification portion, as these I understand. What I don't know is how it's converting frequency into voltage at R1 ?

This circuit was demoed in several Youtube videos, but the operating mechanism is never simply explained. Maybe they understand it so well that they forget that an explanation is needful..

Here's an explanation for a different kind of FM demodulator, and it'd be great if this circuit could also be explained in the simple style of the explanation below:

My simple explanation for a FM Monostable Multivibrator demodulator:

  1. A multivibrator produces a voltage "tick" for every peak of the sinusodial waveform, so higher frequency produces more ticks and vice versa.
  2. When the voltage rectangles coming out of the multivibrator are loaded with a low pass filter, a greater voltage will be seen across the capacitor for denser multivib output and vice versa.
  3. So higher frequency --> denser multivib output --> higher voltage across lowpass filter, which is a biased form of the audio voltage behind the modulated signal.

EDITS

People kindly pointed out that the LC tank's amplitude varies with the frequency.

But why have an oscillator in the first place if all we need is a slope detecting LC tank ? (A slope detecting LC tank + a diode envelope detector to filter out the carrier). Why bother with a Collpits oscillator ? Moreover, what filters out the carrier after the LC tank varied the amplitude ?

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    \$\begingroup\$ Try:electronics.stackexchange.com/questions/479112/… . You also might try reading about regenerative detector. \$\endgroup\$
    – glen_geek
    May 8, 2020 at 1:03
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    \$\begingroup\$ the gain off peak of the tuned circuit is frequency sensitive thus turning FM into AM and the 2nd stage rectifiying the AM \$\endgroup\$ May 8, 2020 at 1:06
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    \$\begingroup\$ You tune it to one side of an FM station then the output voltage increases as the station's frequency swings near the LC tuned frequency and the output voltage decreases as the station's frequency swings away from the LC tuned frequency, converting the FM into AM. It also detects all the clicks and static heard on AM. \$\endgroup\$
    – Audioguru
    May 8, 2020 at 1:34
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    \$\begingroup\$ Daniel the oscillator tracks the signal with regeneration is called an Injection Locked Loop \$\endgroup\$ May 8, 2020 at 2:02
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    \$\begingroup\$ No lookup ILL regenerative rx \$\endgroup\$ May 8, 2020 at 2:13

1 Answer 1

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Even a very small signal fed into an oscillating circuit can cause it to lock to the incoming signal (Injection locking.) This is exploited in this circuit where the oscillation frequency follows that of the incoming signal.

As the oscillation frequency varies and moves away from the resonance frequency of the tuned circuit the amplitude will tend to fall, which in turn cause the supply current of the circuit to change - this results in the audio signal appearing at the headphone. (Slope Demodulation). This is a result of the transistor rectifying the oscillations and altering the bias point of the circuit.

This type of circuit, although simple, has many disadvantages and so is rarely used in serious designs.

There's a paper about the operation at https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=6774035, unfortunately behind a paywall.

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  • \$\begingroup\$ Thanks kindly Kevin (@Kevin White). That's the kind of explanation I was looking for. So the osclillator freq. varies with what's coming in from the antenna, and that causes the LC tank's voltage drop rise or drop. 2 questions: 1. Wouldn't you need a diode env. detector to filter out the carrier from the LC tank's voltage ? 2. I suppose the LC tank is also filtering out other radio stations far away from the tuned to freq ? \$\endgroup\$
    – Daniel
    May 8, 2020 at 2:21
  • \$\begingroup\$ I think this answers my first question: "to fall that in turn cause the supply current of the circuit to change" - so basically voltage division between the load resistor and the LC tank, if I understand - thanks. \$\endgroup\$
    – Daniel
    May 8, 2020 at 2:27
  • \$\begingroup\$ I have some doubt on my last comment: I mean the voltage of the LC still oscillates at high freq. even if amplitude is higher or lower. If the voltage of the load R1 is: Vdc - Vlc it's still going to be oscillating. Wouldn't you need an envelope diode detector to remove the high freq. component across R1 ? \$\endgroup\$
    – Daniel
    May 8, 2020 at 2:34
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    \$\begingroup\$ The transistor in the oscillator rectifies it's own oscillation signal. That's how its amplitude stabilizes. \$\endgroup\$ May 8, 2020 at 3:18
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    \$\begingroup\$ Because the Colpitts (aka Kolpitz in the article) oscillator runs off-resonance peak (they have to in order to oscillate), it makes a handy slope detector but it's still a crappy circuit. \$\endgroup\$
    – Andy aka
    May 8, 2020 at 8:23

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