I agree with both Don and Bill. There are really two issues: In the presences of just noise, the DSP with a wider crystal filter is adequate. The additional noise reduction of a narrow crystal filter is negligible. (QRM is another matter.)
The brain/ear system, at least for many operators, is very good at copying weak signals in noise, even with a broad bandwidth. If there's no QRM nearby, I always find it easier to copy with a larger bandwidth, though anything above 500 Hz makes little difference. (Except for quickly tuning across a mostly empty band.) The 250 Hz bandwidth, in my experience, ALWAYS makes it harder to copy weak signals, unless there is adjacent QRM. For that reason, I prefer to operate with 400 Hz bandwidth most of the time, switching to a narrower bandwidth only when necessary. This seems to contradict signal theory, which says that a narrower bandwidth improves S/N ratio until the filter bandwidth is equal to that of the signal. What signal theory fails to take into account is the matched filter in a CW operator's head. 73, Scott K9MA On Feb 9, 2011, at 7:51 AM, Don Wilhelm wrote: > Ed, > > If there are no strong signals on the band or nearby your receive > frequency, then any filter, even the stock 2.7 kHz is sufficient. The > only purpose of the roofing filter is to keep strong signals out of the > receiver passband so they do not activate the AGC and de-sense the > receiver for the signal you are trying to hear. > > 73, > Don W3FPR > > On 2/9/2011 3:07 AM, Edward R. Cole wrote: >> Let me ask a filter question for a different situation. I am >> interested in receiving very-weak CW and currently have the 2.8 KHz >> and 400-Hz 8-pole filters. Will narrowing the DSP bw down to 200-Hz >> work as well as using a 200-Hz filter? There are no strong stations >> either nearby or on the band. >> >> BTW I am impressed with the 400-Hz filter on the K3 when compared to >> the 400-Hz DSP on my FT-847 (no surprise). >> I find trying to hear extremely weak-CW that narrowing down to 200, >> 100 or even narrower makes the difference. >> >> I left a blank filter space on the main receive for adding a narrow >> SSB filter at a later time. I also have the 13-KHz filter for >> FM/AM. Sub-Rx has 2.8 and 13-KHz. >> >> > ______________________________________________________________ > Elecraft mailing list > Home: http://mailman.qth.net/mailman/listinfo/elecraft > Help: http://mailman.qth.net/mmfaq.htm > Post: mailto:[email protected] > > This list hosted by: http://www.qsl.net > Please help support this email list: http://www.qsl.net/donate.html Scott Ellington Madison, Wisconsin USA ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[email protected] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
