I've looked elsewhere for my answer, but I can't seem to find it.

In AM radio, when you want to pick up a signal, you simply "tune" the receiver to the same frequency as the signal's carrier. But how do you tune the receiver for an FM signal? If the carrier's frequency is always being modulated, how does the receiver know what to look for? Does the receiver look at the carrier's amplitude or something?

6/7/2007 2:00:07 PM

one is high freq (fm)

the other is lower (am)

i guess there is a band to tell it witch way to look higher or lower. and its probably why am sounds like crap.

6/7/2007 2:09:48 PM

i think you've got it backwards..... or something. i'm not sure what your confusion is.

AM = Amplitude Modulated. i.e. All AM radio is transmitted on a single frequency, just each signal at different amplitudes. <-- that doesn't appear to be totally correct, but if they do broadcast at different frequencies, the bands are much smaller than in FM.
I don't know what the actual frequency is for our normal AM radio - http://en.wikipedia.org/wiki/AM_radio - it looks like in the US we use medium wave, somewhere between 520–1,610 kHz

FM = Frequency Modulated, so when you tune your FM radio, you're trying to pick up on different frequencies, regardless of the amplitude, i suppose

Each carrier doesn't actually change their amplitude or frequency - that's not what modulated means. Each carrier has their own, unique (per market) frequency or amplitude. When you tune to 106.1, you're setting your receiver to pick up on the signal w/ frequency = 106.1MHz. Then over at 96.1, they're constantly broadcasting their signal at 96.1MHz
On AM, each carrier transmits at a certain amplitude, but I'm unsure what the radio designation (1610 AM, 860 AM) actually refers to....

[Edited on June 7, 2007 at 2:11 PM. Reason : .]


on further investigation, maybe I'm the one that has it completely backwards.
wiki is always a good place to start



http://en.wikipedia.org/wiki/Frequency_modulation


http://en.wikipedia.org/wiki/Amplitude_modulation

[Edited on June 7, 2007 at 2:16 PM. Reason : .]

6/7/2007 2:10:38 PM

the frequency spectrum of the broadcast signal is centered around the carrier frequency with the vast majority of the power in a narrow band on either side (the bandwidth)

6/7/2007 2:16:55 PM

^So does the tuner look for that portion of the bandwidth with the least amount of power to find the carrier?

6/7/2007 2:18:54 PM

You tell the tuner what the carrier is, that's why there's a tuning knob. Digital tuners though can hunt around to find the best place to actually lock on to.

6/7/2007 2:28:20 PM

the power is going to be centered on the carrier frequency in a 200 khz "window" and the radio is designed only to pick-up the spectrum in that window, if you have a digital tuner, then it knows where the carrier frequencies are (87.9–107.9mhz), but if it's analog, you have to move the window until it's over the frequency band you want

6/7/2007 2:31:16 PM

Quote :

"You tell the tuner what the carrier is, that's why there's a tuning knob."

True.

I guess where I'm getting confused is that I don't know how the tuner "knows" what the range of frequencies is. If you know the range, then you just look at the center and voila, there's your carrier.

Is there a standard amongst broadcasters on how big the bandwidth can be for any given signal, or can they be as big as you want provided you don't overlap on someone else's range?

Actually, I think where I'm getting confused is that I was thinking that the tuner actually "looks" to see if an incoming carrier signal matches the frequency that you select. For AM this isn't a problem because the frequency is always constant, so if you tell the radio to look for a 9kHz signal, it will only pick up a signal that oscillates at 9kHz. But with FM, if you tell it to look for a 100MHz signal, the overall signal won't actually be 100MHz because you keep shifting the frequency. I guess I'm asking how the tuner knows what to look for if it doesn't have a fixed frequency to detect.

[Edited on June 7, 2007 at 2:46 PM. Reason : blah]

6/7/2007 2:42:10 PM

each channel is seperated by 200khz, so if the first channel is at 87.9mhz, then the next channel is at 88.1, and the next at 88.3 and so on. in fm broadcasts, if the bands overlap the receiver will always capture the stronger signal

6/7/2007 2:45:41 PM

Yes, it is a federally mandated bandwidth around your FCC licensed frequency.

So, to restate it, an FM station has its center band, 106.1 FM, which is 106.1 MHz. Someone here said the band was 200 khz across, sounds reasonable to me. At any given time the transmitter is sending only one frequency, but that frequency shifts within the band 106.0 MHz and 106.3 MHz. When the audio being transmitted is at its lowest legal voltage point, the transmitter is putting out 106.0 MHz, when the audio is at its highest legal voltage point, the transmitter is putting out 106.3 MHz. And when silence is being transmitted, the audio is at its center point, often 0V, so the transmitter is putting out a constant 106.1 MHz.

Now, this is not to say the Amplitude does not vary; it is going to vary as you drive under bridges, into the trees, or simply get further away, the average amplitude will fall. On a simple AM radio, you would need to compensate for this by turning up the volume. Of course, with the advent of transistor radios this task is performed automatically on our AM radios so that the volume does not change while the signal gets weaker, just the quality degrades.

But with FM, as long as the detector circuit can distinguish the target signal from the background noise, audio quality will not degrade.

6/7/2007 3:01:28 PM

These sound like questions you would have after just taking ECE 200.

In a basic AM radio demodulator, the receiver doesn't "detect" the carrier signal, it just assumes it's there, and if it is, you get meaningful sound, and if not, you get noise/nothing.

It's the same way with FM. The receiver is going to demodulate whatever is input to it regardless of if anything is there or not.

And sound in the digital format (music CDs) has a range up to 22khz, so FM radio being analog only has to reproduce up to 22khz (i just looked it up and FM radio does up to 15khz). There's some other math involved to factor in stereo sound and transmission efficiency though.

http://www.tpub.com/neets/book12/51c.htm This has a good description of how FM demodulation works, in a basic way.

6/7/2007 3:22:00 PM

^Ok, I see. It just assumes that there's something at the frequency you tune it to.

Sounds simple enough, to the point where I should have thought of it on my own. I've never taken ECE200, though.

6/7/2007 3:28:19 PM

Quote :

"I guess where I'm getting confused is that I don't know how the tuner "knows" what the range of frequencies is. If you know the range, then you just look at the center and voila, there's your carrier."

yeah, as moron said, the tuner doesn't "know" anything - it's not intelligent. It's just a simple electrical filter that picks up electromagnetic waves from the air and passes them through a filter, depending on the frequency that you've selected from the dial, and out the other side of the filter is the range of frequencies that you selected.

It's basically like an equalizer, which you make a crude version of in ECE 212, in the form of a Band Pass Filter, which can be as simple as a couple resistors and capacitors. If you use a potentiometer (variable resistor) in place of one of the resistors, you can adjust the frequency range you allow to pass through, which is exactly like tuning into a specific radio station.
http://en.wikipedia.org/wiki/Band_pass_filter
http://en.wikipedia.org/wiki/Low-pass_filter
http://en.wikipedia.org/wiki/High-pass_filter

6/7/2007 4:13:28 PM

Quote :

"It's basically like an equalizer, which you make a crude version of in ECE 212, in the form of a Band Pass Filter, "

ECE212 is logic design, using wires and IC's only, I sure as hell never made any kind of filters or equalizers.....

6/7/2007 6:05:39 PM

Isn't that first year circuits where you make the filters?

6/7/2007 6:25:18 PM

^^ yeah, you're right..... ECE 211 then? Circuit II, I think it was.
Or that's what it was back in the late 90's. I think they've updated the curriculum since then

6/7/2007 7:33:40 PM

301 is circuits II, IIRC.

6/7/2007 7:34:47 PM

211 circuits 1
301 linear systems

6/7/2007 7:49:29 PM

yeah 211 is all on paper, no projects or building anything at all, but they do teach filters, a little more than in 200.

6/7/2007 8:02:11 PM