billcalley said:
Hi all,
I cannot seem to find any strong explanations for "residual AM",
"residual PM", and "residual FM" in any of my electronic's books or
Online. What are the causes and symptoms of these particular effects?
Any help is most appreciated!
"residual AM" is the unwanted amplitude modulation when you
frequency-modulate an oscillator. This can be because your FM is tuning
outside the bandpass of some element or simply because of
gain-bandwidth product for very wide deviations. You eliminate it by
running the signal through limiter stages.
"residual FM" is the unwanted frequency shift caused as you attempt to
do amplitude modulation. Cause is usually too much coupling of the
modulation into voltage-variable-reactances in the tank. The solution
is usually to add more buffering and isolation between the oscillator
stage and the modulator stage.
"residual PM" is an unwanted phase shift as you amplitude-modulate a
stage or as the input level to a discriminator changes. Solution is
usually better limiting/isolation/bypassing/power supply regulation
(this is often not too different than residual FM).
The above definitions of residual AM and FM are pretty much as
described in old radio texts. Unwanted PM most often occurs in a
comparator stage (boundary between analog and digital).
Tim.