Technical Question About Frequency Modulation

Unanswered Question
Apr 16th, 2007

Does anybody know what kind of considerations are given to the frequencies used in FM, that is the freq's used to represent the user data. For simplicity sake, using a two-level FM model for modulating binary data, a "0" is represented by no modulation of the carrier, and a "1" is represented by changing the frequency.

Questions:

1)Is the new frequency being used allocated as part of the standard?

2)How high above (or below) the carrier is it usually?

3)Is there a possibility of intereference with another signal that is transmitting using the same FM modulating freq of one system.

Sorry if this is worded confusing.

I have this problem too.
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scottmac Mon, 04/16/2007 - 17:01

I'm pretty sure that you would actually use two different frequencies: One to represent a zero, one to represent a mark/one.

That would prevent a loss of modulation to be interpreted as a zero.

Most frequencies, when assigned as "channels" (like FM radio) is actually a chunk of spectrum that represents the fundamental center, plus the sum of fundamental + max modulating signal frequency and the difference of the fundamental and the maximum modulating signal frequency.

When you listen to FM radio, the audio at any given instance in time represents a number that is the difference, the delta, the change of frequency from the fundamental.

By assigning the "frequencies" as "channels," it prevents an interfering overlap.

If you look at the channel assignments for 802.11b or 802.11g, you'll see that each channel uses 25MHz ... but the channels are only 5 MHz apart. That's why channels 1, 6, and 11 are called "non-overlapping" channels. They don't overlap each other, but they will overlap adjacent channels.

More common modulation techniques for data are Phased-Shift Keying (PSK), Quadrature Phase-Shift Keying (QPSK) and variants.

PSK-style modulation shifts the phase of the signal to represent a one or zero, QPSK can represent multiple bits, four positions give you 00, 01, 10, and 11 ... so you are essentially sending two bits with one phase shift ... (kinda) like doubling your throughput.

http://www.howstuffworks.com probably has a pretty decent explanation, give it a look.

Good Luck

Scott

robfollett Tue, 05/22/2007 - 07:43

"Frequency modulation" is a very broad term that means that you are imposing data on the signal by changing the frequency, as opposed to changing the amplitude (amplitude modulation). There are many frequency modulation techniques, but I'll talk about the ones used by 802.11B (actually phase modulation as opposed to frequency modulation) and 802.11A/G for this forum. Feel free to ask if you want to know about any others.

802.11B uses Direct-Sequence Spread Spectrum (DSSS) modulation. Read more about DSSS here:

http://en.wikipedia.org/wiki/Direct-sequence_spread_spectrum

802.11A and 802.11G, although in different frequency bands, both use Orthagonal Frequency Division Multiplexing (OFDM). Read more about OFDM here:

http://en.wikipedia.org/wiki/Orthogonal_frequency-division_multiplexing

--

Rob Follett

FCC ID #PG00020467

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