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Old 3rd Aug 2021, 1:57 pm   #1
AidanCroft
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Default FM tuned circuits, ratio detection etc

Eh up,

I've tried researching for myself and have hit a dead end. I'm trying to learn the principles of AM/FM demodulation.

If I'm right, I believe it starts with a resonant circuit that mixes the received frequency with an oscillator to produce an IF that in turn is amplified. In AM, a detector/demodulator only allows the positive peaks to pass, which is then smoothed and amplified to the speaker.

This is the SAME in FM I believe except the frequency deviation from the main carrier frequency represents and output Voltage, so recreating the signal? All correct so far?

My only confusion is, if the variable capacitor sets up a resonant circuit for one frequency only, the carrier frequency, where is the deviation coming from fir the ratio detector to make use of?

Kind regards,

Aidan.
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Old 3rd Aug 2021, 4:00 pm   #2
duncanlowe
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Default Re: FM tuned circuits, ratio detection etc

Although you are tuning to 'a frequency' you are tuning the centre frequency, plus a bit either side. A 'perfect' tuned circuit wouldn't have this, but then there is no such thing as a 'perfect' tuned circuit. So enough of the frequencies either side of the centre frequency get through to see the deviation either side of the centre frequency, and demodulate it. The amount 'either side' of the centre frequncy is the bandwidth. For voice transmaission we tend to keep the bandwidth quite narrow (narrowband FM) but broadcast tends to allow a wider bandwidth (wideband FM).
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Old 3rd Aug 2021, 4:03 pm   #3
AidanCroft
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Default Re: FM tuned circuits, ratio detection etc

Quote:
Originally Posted by duncanlowe View Post
Although you are tuning to 'a frequency' you are tuning the centre frequency, plus a bit either side. A 'perfect' tuned circuit wouldn't have this, but then there is no such thing as a 'perfect' tuned circuit. So enough of the frequencies either side of the centre frequency get through to see the deviation either side of the centre frequency, and demodulate it. The amount 'either side' of the centre frequncy is the bandwidth. For voice transmaission we tend to keep the bandwidth quite narrow (narrowband FM) but broadcast tends to allow a wider bandwidth (wideband FM).
Ahh brilliant!! Thanks for educating me. Knowing that now makes a lot more make sense.

Are my other general ideas on tuning and demodulation correct?

Aidan.
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Old 3rd Aug 2021, 5:15 pm   #4
Radio Wrangler
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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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Old 3rd Aug 2021, 7:21 pm   #5
MotorBikeLes
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Default Re: FM tuned circuits, ratio detection etc

Fifty years ago I acquired an oldish but postwar Cossor booklet entitled FM Explained (or something similar). I found that very helpful. Maybe one of our Cossor experts has a copy.
I think I still have mine, but I can't find it on the bookshelf next to me. It may be in a filing cabinet or anywhere.
Les.
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Old 3rd Aug 2021, 7:49 pm   #6
G6Tanuki
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Default Re: FM tuned circuits, ratio detection etc

The classic Foster-Seeley and Ratio-detectors are sensitive to both frequency and phase: simplistically they use two tuned-circuits which are either magnetically-coupled or use a combination of magnetic- and capacitance-coupling.

When there's just an unmodulated carrier present the combination of magnetic-and-capacitance coupling produces no audio component [though it can provide a DC component for use in AGC and AFC].

This is essentially a state-of-balance.

When the frequency of the carrier is wobbled by FM modulation, this means it moves out of the peak of the discriminator's tuned-circuits and the result is an instantaneous imbalance, the extent of which follows the audio modulation of the carrier - so the discriminator produces an audio signal according to the extent of the imbalance.

AM signals - typified by impulse noise from lightning or unsuppressed car-ignitions - are limited in the IF stages of a FM receiver - and even if they make it past this to hit the FM discriminator they only serve to 'push up' the average level of the signal on each pulse, rather than having any either-side-of-the-response-curve component, so they don't present themselves at the audio output of the discriminator.
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