Barry N1EU wrote:
> I find that the most noticeable impact of a filter's group delay
> characteristics is how they pass impulse noise, ie. lightning strike
> QRN. The poorer filters tend to turn the short impulse into longer
> roaring noise. The difference between filters can easily make the
> difference in being able to pull out weak cw signals or rendering them
> unreadable.
Yes, it can stretch a pulse but the real impact of filters with poor
group delay is when using digital modes, e.g. PSK31, packet, PACTOR,
etc. A filter with poor group delay characteristics can make an
otherwise good signal impossible to demodulate.
If you have every found that you can copy a PSK31 signal properly with a
wide filter but not with your narrow filter you have been bitten by poor
group delay in your narrow filter.
If you think about modulation products (sidebands) you can imagine the
different frequencies (spectral lines) progressing through the filter at
different rates resulting in phase errors. The resulting baseband
waveform after demodulation will no longer be an analog of the original
baseband modulating waveform.
73 de Brian, WB6RQN
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