Suppose a 1 MHz signal is frequency modulated by a 1 kHz sine wave. The
frequency deviates over some frequency range, for example +/- 5 kHz. If we
put that signal into a times-ten multiplier the FM signal is at 10 MHz and
the frequency deviatioon is ten times greater (+/- 50 kHz). However, the
*rate* at which the 10 MHz signal traverses the greater frequency deviation
is still 1 kHz. That does not change and that is what the FM detector output
delivers.

Bill W0IYH

"Joel Kolstad" wrote in message
...
I was over on comp.dsp exposing my ignorance the other day when it
eventually
dawned on me that a frequency multiplier will, in the frequency domain,
just
convolve whatever the input signal is with itself. This got me to
thinking...
why is it that frequency multipliers work as well as they do for something
like FM? Assuming a sine wave modulating signal, the FM spectra is a sum
harmonics with amplitudes dictated by a Bessel function; frequency
multiplying
this would seem to add new harmonic content to the mix besides just
doubling
the frequency of what's already present. So... does it turn out,
mathematically, that frequency multiplying an FM signal just so happens to
end
up what nothing more than a "frequency scaled" spectra of what was
originally
present? Or is some amount of distortion added in the process (assuming
perfect mixers used as the frequency multipliers and the DC component of
the
mixers' outputs removed).

I've been told that, in general, frequency multiplier can be effectively
applied to most any modulation scheme that has a reasonably constant
envelope,
e.g., FM, PM, FSK, even QPSK. Is this generally accepted knowledge?

Thanks,
---Joel Kolstad