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can am detector be used for fm radio reception?

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BkraM

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Hi all,

I'm trying to figure things out about FM demodulation.
A first step, simple option for FM demodulation should be a slope detector.
As such i would think that a AM detector, slightly off-tuned to a FM radio station would process the FM signal to audio.
I'm trying to get some audio in the 100 MHz range from my simple AM detector (tank, diode, amp) but am not able to receive anything in at this band.
Am I wrong that it should work (at least a bit)?

Thanks,
 
You would need to be very close to the transmitter for this simple "crystal set" to work.... Also the slope of the skirt of the a simple LC tuned circuit tuned off to the side of the transmitter frequency is not likely to be steep enough to create much slope-detected audio.
 
A simple (but somewhat finicky) circuit for FM (or AM) reception is the Super Regenerative receiver.
I built this single tube circuit many years ago that picked up strong local FM stations reasonably well (although not with great fidelity). Still have it in my attic.
 
FM radio cancels amplitude interference and has very low distortion. An AM slope detector produces amplitude interference (snaps, crackles and pops) and a lot of distortion.
 
I'm trying to get some audio in the 100 MHz range from my simple AM detector (tank, diode, amp) but am not able to receive anything in at this band.
Am I wrong that it should work (at least a bit)?

Yes, that theory will work, but as Mike pointed out you need a filter (tank) with a sharp enough slope, or the amplitude variation will not be great enough to detect. You will then have to tune it very slowly and carefully to be dead on the slope only. You will need a strong local FM station.

We used to connect the vertical display (amplitude) output of our spectrum analyzer to an external, amplified speaker and a long wire to the input so to listen to FM stations in the lab by using the slope of the resolution bandwidth filter as a detector. The frequency span is set to zero Hz so that the filter is not sweeping. The bandwidth (and thus the slope) of the resolution bandwidth filter is selectable. It sounded fairly decent. We could tune in stations from DC to light. Tuning was easy, of course, because the frequency and detected signal is displayed on the screen. Some spectrum analyzers even have FM detectors and phone jacks (and even small speakers) built in.
 
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In Google, there are a few FM Crystal radio projects, here is one: https://solomonsmusic.net/FM_CrystalRadio.html
It picks up only a few very strong local FM stations. It feeds an audio amplifier that has a very high input resistance.

But all FM radio stations use pre-emphasis that boosts high audio frequencies and all FM radios have de-emphasis that cuts the high audio frequencies back down to normal which also cuts hiss. Then the simple FM Crystal Radio without de-emphasis will have way too much treble sound.
 
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