David Woolley wrote: >Ian White GM3SEK wrote: > >> >> However, contesters are always looking for an extra edge, and a true >> 250-270Hz 8-pole filter with a steeper transition from the passband into >> the stopband would be the next step to try. >> >At that sort of bandwidth you are going to be clipping a non-trivial >amount of sideband power. Are you sure that a brick wall filter would >be a good idea; I would wave thought it would cause significant dispersion. >
That theory only applies to the copy of weak RTTY signals against a background of noise. But when the main problem is QRM, the best *available* copy is obtained by reducing the bandwidth and accepting a small reduction in accuracy. The recommendation for a 250-270Hz filter is based on many years of practical experience in heavy contest QRM, starting with different combinations of cascaded filters in the FT-1000MP, and then moving on to different combinations of roofing filters and DSP in the K3. The measurable performance parameters have been certificates and a modest amount of silverware. With off-the-shelf roofing filters in the K3, the quality of RTTY copy in extreme QRM was inferior to the FT-1000. The 400Hz filter let in too much QRM which was routinely triggering the hardware AGC, while the 200Hz 5-pole required manual fine tuning for each new caller which made it unusable for serious contesting. With the modified 270Hz 5-pole, the performance of the two radios is now about the same... so the next logical step forward would be a 250-270Hz 8-pole, a "gaussian to 6dB" design with improved roll-off outside the passband. -- 73 from Ian GM3SEK http://www.ifwtech.co.uk/g3sek ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:Elecraft@mailman.qth.net This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
