Chapter Four: Synthesis
6. Principles of Audio-rate Frequency Modulation | page 3
Calculating sideband frequencies
Cƒ ± nMƒ [n=0,1,2,3...]
OR (for those with math anxiety)
the carrier frequency (Cƒ) plus and minus all the integer multiples of the modulating frequency (Mƒ)
OR (for those with really serious math anxiety)
Cƒ, Cƒ + Mƒ, Cƒ - Mƒ, Cƒ + 2Mƒ, Cƒ - 2Mƒ, Cƒ + 3Mƒ, Cƒ - 3Mƒ, etc. to infinity and beyond
For example, a carrier frequency of 400 and a modulating frequency of 50 will produce the spectrum listed below for the first three sideband pairs (n=0 to n=2)
| integer multiple |
fm spectrum | math to find spectrum |
| n=0 | 400 Hz | 400 + (0 * 50) |
| n=1 | 450 Hz, 350 Hz | 400 + (1 * 50), 400 - (1 * 50) |
| n=2 | 500 Hz, 300 Hz | 400 + (2 * 50), 400 + (2 * 50) |
A graph of this example's sideband pairs, related by color, appears below:
Another way of calculating sidebands, useful when the carrier and modulator are maintaining a constant frequency relationship is through a ratio of carrier to modulating frequency (C:M). For example, a Cƒ of 100 and an Mƒ of 200 would produce a C:M ratio of 1:2 (click here to see how to reduce C:M ratios to their "normal form"). We'll see below that integer ratios that have a carrier value of 1 have certain properties. We could calculate the upper sidebands for this ratio in relation to the carrier frequency as C+M, C+2M, C+3M, C+4M, etc. For our 1:2 example, the first upper sideband would be 1+2=3, the second would be 1 + (2 * 2)=5. If you worked this out in Hz, you would quickly come to the conclusion that these are the odd numbered partials of the carrier frequency. We calculate the lower sidebands similarly as C-M, C-2M, C-3M, C-4M or in our 1:2 example, -1, -3, -5, etc.
