[Elecraft] group delay distortion

[Jack Smith]

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:
> Hi all:
>
> Okay, I'm not an engineer.  Just what is group delay distortion?
>
> Please reply direct, unless you feel that the group could learn 
> something.
>
> de Doug KR2Q
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