Doug:
Group delay is a measure of phase shift (and hence time delay) for a
signal as it passes through the filter. Consider a signal swept through
the bandpass of a filter. If the signal always takes the same time to
pass through the filter, regardless of where it is with respect to the
passband, it would have a flat group delay.
Almost all filters, however, have a delay that changes with frequency;
if we consider the delay at the center of the filter as 0, the delay is
usually greater at the edges of the filter.
Some filter designs have a relatively flat group delay and others have a
highly variable group delay. In general filter types with steep skirts
have the worst group delay variation and vice versa.
So, why should you care about group delay?
Suppose you have a SSB signal passing through a filter with poor group
delay. The various spectral components that comprise the SSB voice
signal will enter the filter in the correct time and phase order but
when they leave the filter, some frequencies have been delayed (or phase
shifted) with respect to other frequencies. The result is distortion of
the signal.
A CW signal has transients at transition points, key-up and key-down and
to the extent those are differentially delayed through the filter, the
result can be strange sounds.
I'll send you some group delay data on filter's I've built. Can't send
it through the reflector, of course.
Oh, yes, I've used the term phase shift and time delay more or less
interchangeably. That's because they are interchangeable in this case.
Jack
DOUGLAS ZWIEBEL wrote:
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