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fm demodulation circuit

akbarza

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hi
this is a FM receiver schematic( I saw it in a app on a android mobile).
please explain to me:
1) what is the role of Q1? we know that LC Tank pass some frequencies and forbid(attenuate) other frequencies. How does Q1 act in each two frequency groups? what does happen if we omit Q1?
2) in each two frequency group, how does Q2 act?
3) how is demodulation of fm input signal done?
4) what are the role of c1, c2 and c3?
fm_demodulation.jpg
and why are these values choosen for them?
when there is not input fm signal, do the transistors consume power? if yes, how fix it?
5) can you introduce to me a reference or references for more understanding of such circuits?
thanks
 

betwixt

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Research "superregenerative" receivers. They are not truly FM, they are AM but can resolve FM to some degree by offsetting the tuning point so the frequency deviation (modulation) of the FM signal moves closer or further away from the center point.

Expect performance of that design to be very poor and audio quality bad but it will work.

C1 & C3 are for audio coupling. C1 from the receiver to the volume control and C3 with it's larger value to feed the headphones, without them the DC present on the signal would upset the amplifier and make excessive current flow through the headphone. C2 is to try to stop some of the instability in the amplifier by 'shorting out' the higher frequencies although technically this is not the best way to do it.

Brian.
 

KlausST

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Hi,

IMHO: Don´t waste your time with circuits from such unreliable sources.

Maybe - if you omit the short circuit at the output - you may hear something. But don´t expect any really useful.

To your questions:
* there is a brief - not 100% correct description on their website.
* read the LM386 datasheet (it´s a very old IC of bad quality. Nothing one would use for serious circuits)

***
There are tons of good information
* how FM transmitter work
* how FM receivers work
* of simple, good discrete FM reciever circuits (with description)
* of simple, good receiver ICs, with datasheets, application notes, design notes

Good, reliable informations come from semiconductor manufacturers, universities, engineers with good reputation.

Klaus
 

danadakk

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