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" Detecting Incidental FM
One final observation that we may make much easier in the frequency domain
is to determine if the signal generator has simultaneous AM and FM
modulation. Almost all signal generators before the era of phase lock and
DDS techniques have some degree of incidental frequency modulation that
occurs simultaneously with the desired amplitude modulation.

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Simultaneous AM and low level incidental FM will reveal itself in
different amplitude of the upper and lower sidebands. To illustrate this, I
intentionally simultaneously modulated my Boonton 102D signal generator with
1 KHz, setting the AM level at 30% and FM level corresponding to a deviation
of 100 Hz. This is a low amount of FM, but it shows up as a quite
noticeable difference between the upper and lower sidebands. You will note a
very small difference between the two sidebands in the 5% AM modulation as
well, perhaps 1 dB or so. This is the result of a very small unwanted
simultaneous AM and FM modulation."



Marek