> Having used three different K3's equipped with 4 different > sets of 250 & 400 hz 8 poles in overload city major contests, > the usefulness of these filters is belied by the apparent > closeness of numbers.
I'm happy that you are finding success with the 400/250 filter pair but given the published specifications I suspect your results have more to do with the DSP performance than the actual width of the 250 Hz filter. There is so little difference in the published curves for the two filters (150 Hz at -60 dB) that roofing filter performance alone can not be responsible for the differences performance you describe. If one is looking for improved "narrow CW" performance, the 400/200 combination is a more effective and cost efficient pairing. > Anyone who asks us what filters to buy for CW contesting, we > tell them 400/250 8 pole set to 450/350. Tried and true, not > theory. > > W4TV and I will just have to agree to disagree. I don't think there is any disagreement ... unless you have actually tried a 400/200 pair set to 450/250 and are trying to say that the 250 Hz filter is tighter and suffers fewer hardware AGC effects than the 200 Hz filter. If you try to make that point, there are several others including W4ZV, who will disagree with you. 73, ... Joe, W4TV > -----Original Message----- > From: elecraft-boun...@mailman.qth.net > [mailto:elecraft-boun...@mailman.qth.net] On Behalf Of Guy > Olinger, K2AV > Sent: Tuesday, August 04, 2009 10:14 PM > To: ni0c; elecraft@mailman.qth.net > Subject: Re: [Elecraft] [K3] Elecraft roofing filters > > > Having used three different K3's equipped with 4 different > sets of 250 & 400 hz 8 poles in overload city major contests, > the usefulness of these filters is belied by the apparent > closeness of numbers. > > In practice we have found that associating the 400 with a DSP > width of 450, and the "250" with a DSP width of 350 has been > extremely useful, however "too close by the numbers" that may > look on paper. > > We use 450 for running for as long as it may last, and when > the inevitable 30 over 9 crowder squeezes down on us, I > reduce to 350 and if that isn't enough, *ADDITIONALLY* shift > the center away 50 hz more. This combination plus the noise > blanker for key clicks has worked extremely well. (Having 50 > Hz granularity on the CW shift/widths would be *SO* useful here...) > > I have measured the combined (roofing+DSP) drop on the > steepest part of the skirts at ~12 db per 10 Hz with these > two 8 pole filters. So bringing in the skirt "only" 30 or 40 > Hz is enough to push down the crowder quite a bit, usually > well out of hardware AGC, without narrowing the listening > window to the point of missing QSO points and multipliers > from all the inevitable off-frequency callers. > > Anyone who asks us what filters to buy for CW contesting, we > tell them 400/250 8 pole set to 450/350. Tried and true, not theory. > > W4TV and I will just have to agree to disagree. > > 73, Guy. > ----- Original Message ----- > From: ni0c > To: elecraft@mailman.qth.net > Sent: Thursday, February 05, 2009 4:47 PM > Subject: [Elecraft] [K3] Elecraft roofing filters > > > W4TV wrote: > "The difference between the 250 and 400 Hz filters is not > enough to be worthwhile (approximately 375 Hz vs. 430 Hz). " > > Joe, what is your source for these numbers? I've seen numbers > like this mentioned previously on the reflector. > > I recently tested my K3 CW roofing filters using the XG-2 generator > at 7040 KHz (50 microvolts input) and KS7D's nice software package, > "K3 Filter Tools." > > Here's what I came up with, with AGC off and with the DSP > bandwidth set at least as wide as 900 Hz for all tests (to isolate > the effects of just the crystal roofing filter): > > 200 Hz, 5-pole: -6 dB BW = 210 Hz; -30 dB BW = 430 Hz > > 250 Hz, 8-pole: -6 dB BW = 260 Hz; -30 dB BW = 500 Hz > > 400 Hz, 8-pole: -6 dB BW = 380 Hz; -30 dB BW = 580 Hz > > Note the uncertainty in each of the bandwidths above is > plus or minus 20 Hz, because I ran these sweeps in 10 Hz > increments to save time. (I'll repeat these tests using a > 1 or 2 Hz increment, when I have some spare time.) > > These tests on my filters (as well as my experience by ear) > indicate there is an appreciable difference in the 250 and > 400 Hz 8-pole filters. I've always wondered, too, about > manufacturing tolerances on narrow crystal filters. > > 73, > Chuck Guenther NI0C > > > > > > > > > ______________________________________________________________ > 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 > ______________________________________________________________ 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
