Since I have gone through the optimization of four different radios for WSJT use, I can make some suggestions.
As it turns out the same requirements apply to the JT65 modes used for EME so you only need to have one setup for any WSJT mode used for any propagation mode. FSK441, JT65 A,B & C, and the new JT4 modes all require a 2.7 kHz or greater bandwidth for optimum performance both on TX and RX. Although the software can compensate to some degree for passband amplitude variations, the best performance will be obtained with a perfectly flat passband for both TX and RX. Even though the bandwidth of these modes is less than the 2.7 kHz specified, you need to have the wide filters to allow for doppler, radio drift, frequency errors, etc. There is no downside to using the widest filter possible for these modes as the detection bandwidth inside the software is on the order of 1hZ! Many EME operators use a SDR radio wide open at 96 or 192 kHz width and the beta software which will decode multiple signals simultaneously in that passband. Larry - W7IUV --- Bill W5WVO <[EMAIL PROTECTED]> wrote: > All this leaves me wondering: What will be the best > filter configuration (both > TX and RX) for running FSK441 on meteor scatter? In > case you're unfamiliar: > > "FSK441 uses four-tone frequency shift keying at 441 > baud. The frequencies of > the audio tones are 882, 1323, 1764, and 2205 Hz. > Each encoded character uses > three tone intervals and therefore requires 3/441 > seconds (approximately 2.3 > ms) for transmission. FSK441 accommodates an > alphabet of 43 characters." > -- from the WSJT manual by Joe Taylor, K1JT > > Bill W5WVO > > > Don Wilhelm wrote: > > Larry, > > > > You are quite correct that all filters are not > created equal. For > > digital modes, the group delay is just as > important (if not more > > important) than a flat passband and steep filter > skirts. In fact, > > those filters with steep skirts often sacrifice > group delay to > > achieve the steep skirts - but I am generalizing > here, and that may > > not be a universal truth. > > > > Unfortunately, group delay plots are not common > for filters since > > steep skirts seem to be the 'criteria of choice' > for most amateurs. A > > Gaussian to 6 dB filter has a nice rounded nose > and gentle skirts, but > > has a great group delay characteristic, OTOH, Cohn > filters usually > > have great skirts and poor group delay in the > passband. Group delay > > will make a difference in the ability to decode > digital signals. > > > > 73, > > Don W3FPR > > > > Larry Molitor wrote: > >> --- "Joe Subich, W4TV" <[EMAIL PROTECTED]> > wrote: > >> For a died in the > >> > >>> wool > >>> RTTY DXer, a steep sided 270 - 300 Hz filter > would > >>> be nice > >>> instead of 370 Hz from the "250 Hz" filter. > >>> > >> > >> Over the years I've spent a lot of time looking > at > >> this issue. My interest is primarily digging weak > RTTY > >> DX out of the noise but I do occasionally get > into a > >> contest. > >> > >> I've played with a lot of radios and filters but > most > >> of my real testing was done with a FT-980, a > TS-2000, > >> and a FT-2000. > >> > >> While I have seen good weak signal performance > with a > >> 250 Hz filter, it was the exception not the rule. > As > >> an example, the improvement on the FT-980 going > from > >> the stock SSB filter to the dual CW filter was > >> dramatic. But going to the 250 Hz CW filter lost > about > >> 6 dB in ability to properly decode weak RTTY > signals. > >> It was even worse on signals that had polar > flutter. I > >> no longer have the plots of these FT-980 filters > but > >> as I recall, the 250 Hz filter was a bit peaky in > the > >> middle and had poor group delay characteristics > >> extending well in from the corners. > >> > >> As you say Joe, the determining factor is the > passband > >> ripple/group delay. In a typical bandpass filter > the > >> group delay goes to heck at the corners. But the > shape > >> of the "corners" varies from filter to filter. > I've > >> run a number of "ham filters" on a network > analyzer > >> and plotted group delay. Seems like no two > filters are > >> the same even if the same part number. This I > believe > >> is due to ham filters being so cheap and > manufacturing > >> process control being minimal to keep the sell > price > >> down to what we can afford. > >> > >> So I would say, if you have a 250 Hz filter, no > matter > >> where it's placed in any radio, give it a try on > very > >> weak signals. Switch between a wider filter and > the > >> narrow filter and see if there is any > degradation. If > >> not, use the narrow filter. > >> > >> Other wise, plan on not using any filter less > than 300 > >> Hz in passband width (note - this is NOT the 6 dB > down > >> width!!! very important!!) if you want optimum > weak > >> signal performance. > >> > >> > > _______________________________________________ > > 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 > > ____________________________________________________________________________________ Looking for last minute shopping deals? Find them fast with Yahoo! 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