Electronics Forum / Basics / November 2005

On 11/28/05 11:55 AM, in article
1133207727.457143.258890@g43g2000cwa.googlegroups.com, "billcalley"

Bad design, I imagine.

Don

Gee, I can't imagine that Google would fail in this arena.

Anyway, residual AM means that one gets some amplitude modulation of a
signal when the intent was FM or some other type of modulation.  This can
happen when power supplies are not stiff or when the modulators (the
circuits that combine the intelligence signal with the carrier signal) are
marginal.

Residual FM means that one gets some frequency modulation of a signal when
the intent was AM.  This one is easier as it usually is caused by non-stiff
power supplies.

QUAM modulators have some interactions and subtle effects that I cannot help
you with here.  QUAM is an acronym for Quadrature Amplitude Modulation.  I
am certain that you can Google for this.

Good luck!

"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.