AM radios were easy to understand. The strength of the signal goes up and down, and the audio follows. FM radio is a little more difficult. [AllAmericanFiveRadio] has an old tube FM set and takes us on a tour of how the FM discriminator works. You can see the video below.
The first step is to look at the IF signal on the scope. It is hard to see, but the frequency is changing, and that’s the basis of modulation that the discriminator has to resolve.
The next step is to feed a fixed signal into the IF tuned circuits. The scope shows that at the center frequency, the signal passes through with no issue. However, as the frequency changes, the tuned circuit attenuates the output, converting the frequency change into an amplitude change. Unsurprisingly, once you have the analog change, you can demodulate it like an AM signal.
There is a bit more, of course, but the basic idea is just that simple. The video redraws the tube schematic with diode symbols, and the explanation is clear.
You should know that there are several other ways you can decode FM. Also, of course, there are many other ways to modulate a carrier with information.
In principle, any AM radio is capable of demodulation of FM signals, by using edge demodulation – that’s slope detection in English?
That’s a technique which CBers in Europe occasionally had used when FM was being freshly introduced.
Some higher-end AM CB radios had a frequency fine-tuning knob that could be re-purposed to receive an FM signal.
So the AM oldtimers could at least listen to FM conversations.
Their old CB radios were still good as receivers, also.
The trick also being being useful for CB to medium wave converters (Monacor CB-705 etc).
These were little boxes that converted that 27 MHz CB band segment to 526 kHz to 1606 kHz, for use with an ordinary AM car radio.
Yes, I first encountered that when I was a teenager and used my Dads AM/FM/SW/Air receiver. I was picking up hams at the top end of the air bad, and as neighbor who was a ham explained how slope detection worked.
We have FM mode CB in the US now but it hasn’t caught on. I guess the idea is it sounds better, doesn’t need quite as much fiddling with the squelch control, etc. But AM still rules here, the compatibility is difficult to unseat it. And the better range is pretty handy out.
That said my next radio will have and FM mode … Just in case. So it’s a great feature for getting us to but new radios.
I feel like GMRS already serves average people better than CB and the FM mode is a bit redundant when more people are on GMRS/FRS anyways, especially in denser populated areas where the range isn’t and issue and the large number of channels is a big advantage.
Hi! I heard about the news and think it’s neat. I mean, now have all three modulation types on both sides on the pond. AM/FM/SSB.. That maybe makes international contacts happen more often now.
Up until 2011, SSB (12w) wasn’t being officially available on CB in my country (DL):
We had FM (4w) on 80ch, but AM (1w) merely on the 12 original channels (4-15).
The plain CEPT radios in EU were all 40ch/FM 4w for legal reasons.
Some EU member countries had odd their spaced national channels, too, because of historical reasons.
Anyway, I think FM can be neat for digital stuff with higher bandwidth.
We had Packet-Radio over here in my country since 1994.
SSTV (analog really) was being allowed later on, too. Stations from Italy used that mode often, I believe. The company Zetagi (linear PAs etc) is from Italy, too.
Then we got voice repeaters (parrots) and real repeaters since a few years.
The repeater thing is interesting, many newer CB radios have relay shift feature. As with 10m radios.
Question:
Imagine an AM transmitter that is fully synchronous and discrete. It would modulate so that each full wave cycle is transmitted at a constant amplitude. In other words, there’s some digital sampling system that makes sure the modulation value doesn’t change in the middle of the cycle but only in discrete steps at each zero crossing, which would mean each transmitted cycle is a nice pure sine wave without distortion – only the amplitude changes.
What would that do to the transmitted signal spectrum? What happens to the sidebands?
Ugh. Good question. Sounds a bit like a “Speech-Processor” (’70s terminology).
It sort of compresses the RF signal for clearest modulation? Or something along these lines? 🤷♂️ Sorry, I’m just a CBer and SWL when it comes to such things. 🤦♂️
That could be described as a coherent amplitude shift keying scheme. You can’t eliminate the bandwidth; your signal itself isn’t discontinuous in the ideal case but its derivatives still are – or if you rather, the sine waves you’re using are truncated and truncated sine waves don’t have the same properties as you’re used to with continuous sine waves. They’re only pure on a graph, basically.
https://ham.stackexchange.com/a/9446
Morse code actually takes a lot of bandwidth for the clicks that occur if you don’t ramp up and down the amplitude over a fraction of a second every time you switch.
What I always thought was cool is the concept of synchronous detection on the reception side. Radios in avionics use such high quality receivers.
Audio quality is near FM with such air radios.
Speaking of AM, I think it’s neing underrated these days.
The use of a carrier wave makes it easy to “lock on” to a signal, to use an AFC (automatic frequency control).
What’s also cool is its non-destructive nation. Signals can overlap without even out each other. A faint, distant signal can still be heard “in background”.
I think that’s something that FM advocates seem to forget sometimes .
The “primitive” AM has its qualities.
SSB, the most beloved modulation in amateur radio, is a relative to AM.
It’s an AM signal with one side band its carrier wave being removed.
Personally, I wished SSB was being used different these days.
instead of surpressing the carrier completely, it would be more wise to just lower it.
That way, it would still be possible to more accurately lock onto a signal.
All those mickey mouse voices wouldn’t be an issue anymore.
Tuning to a digital station on shortwave would be less error prone, too.
Also interesting would be DSB, double side band. That’s AM without a carrier, essentially.
In a modified form, with two different signals in each side band, an amateur station could send video (say 8s SSTV) and voice simultaneously.
Or transmit in stereo sound, that would also be cool.
Too bad those experimental times are mostly over. SDR technology is literally predestined for this.
Alas, radio amateurs nolonger seems to so open-minded or risk-loving as they were in the 60s-80s.
No offense to our current generation, it’s doing fine. It’s just too tame.
Back then, by contrast, they were still foolhardy enough to ignore band recommendations from time to time in favor of progress, in the name of science.
They played with bigger bandwidths, illegal power levels, used non approved modes etc.
I heard stories about this from my father.
The unofficial side of ham radio was very different to what’s being told by history books.
These were really cool dudes back then, sometimes being foolish in a good way. But that’s another story.
Amplitude companded single side band keeps part of the carrier for the receiver to lock onto. It never caught on in ham radio though.
An AM transmitter that modulates only at zero crossings with constant amplitude in each cycle, while intriguing, wouldn’t function as a typical AM transmitter and would have interesting effects on the transmitted signal spectrum. The lack of sidebands implies the absence of information transmission. Traditional AM relies on sidebands carrying the modulating signal’s information (audio in case of radio). Without sidebands, the receiver wouldn’t have anything to demodulate and recover the original information.
I believe Fourier analysis would still show that signal using typical AM bandwidth (2x modulating signal). Although, I think this may suppress the carrier similar to DSB-SC. But now I’m curious and I’ll have to model it.
“resident frequency” ??
The one can adjustment you did not want to touch to get peak signal like other IF cans. I remember well. Shirt pocket AM/FM transistor radios.