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 _______________________________________________ Elecraft mailing list Post to: Elecraft@mailman.qth.net You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com
