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Old 3rd Aug 2021, 5:15 pm   #4
Radio Wrangler
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Join Date: Mar 2012
Location: Fife, Scotland, UK.
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Default Re: FM tuned circuits, ratio detection etc

Don't try to learn both AM and FM at once. There are enough similarities and differences to trip you up into a huge jumble of confusion.

Start with AM.

Put a microphone into an audio frequency range spectrum analyser and talk/sing/play music into it. The analyser will show you the spectrum of audio frequencies of what you're doing.

Put the microphone into an AM transmitter and make no noises. Use a radio frequency spectrum analyser to look at the output of the transmitter and you'll see just s single output at the frequency it is transmitting on. A single narrow spike at just that frequency, and nothing anywhere else.

Now talk/sing/play again and look at the RF spectrum closely and you'll see two replicas of the microphone's audio spectrum added. butted up to the carrier frequency. One is the right way up in frequency terms, but the lower one is reversed. Note that they make a symmetrical picture around the carrier. Note that the audio spectra jiggle around as you talk/sing/etc, but the cattier spike remains stubbonly constant.

So a modulated AM signal takes up some width on the frequency scale, and the width needs to be twice the fighest audio frequency you want to send.

Tuned circuits are ideally very very narrow, of infinitesimal width. An ideal one would strip your audio modulation off and just pass the carrier, and it'd be rather hit and miss about being tuned accurately enough to get the carrier.

But tuned circuits aren't ideal at all. They let a finite width through. At the intermediate frequency of a typical AM radio some sets do the best they can to just be narrow enough, others unless tuned craftily to widen them can be too narrow and muffle the higher audio frequencies.

After the wanted signal has been filtered and amplified, it needs to be demodulated. Something has to look at the level of signal in the IF, and turn it into a representative DC voltage. Well baseband voltage as it will wander to track the microphone voltage.

So you have to take onboard the concept that modulated signals have 'sidebands' and that these give it width in terms of frequency.

Receivers need to themselves be wide enough to pass this unmutilated, but not so wide that they let other stations in at the same time.

The paths signals take to your aerial can fade up and down in loss, so your signal level wanders up and down slowly. Your receiver needs to adjust its own gain to compensate to give you a nice steady audio level. So there is an automatic gain control circuit adjusting the receiver gain to keep the carrier component (DC from the demodulator) constant.

I'll cover FM when I've more time... got to go out and school some horses.

David
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Last edited by Radio Wrangler; 3rd Aug 2021 at 10:52 pm. Reason: give not five!
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