Hi Don, Every now and then on this subject, I have to throw in this rather significant (IMHO) caveat on all this talk about how good the DSP bandwidth is. I have an old MP, INRAD 8 pole filters in 8 and 455 IF's, that has selectivity just barely matched by a K3 with 8 pole roofing filter AND DSP set for skirt alignment. The DSP by itself is nowhere near the MP's selectivity, sorry. Not close.
There is a point in CW contesting, where if you want to continue on a run frequency, you will have to be able to deal with a MUCH louder signal just a few hundred Hz up or down, and try to copy a MUCH weaker signal in band. And since the K3 is so clean in matters intermod, it really IS possible to deal with a 35 over 9 signal just up and copy an S1 signal in band, a mere 80 dB difference. IF you also want the desired signal to be 20 dB louder than the undesired signal in the audio, you are trying to create a separation of 100 dB !! IF you also want to be able to hear the forever present off-frequency callers, who to be kind, may actually be physically incapable of closely matching two tones (quite common actually), or to not be kind, don't have enough sense to listen first off a packet spot... if all of these, you will NOT be able to narrow down to super narrow to get rid of Mr 35over9 without giving up on all the +/- callers. Once you have figured out that 1/4 of all your contacts are "off frequency" then you have to maintain a barely adequate bandwidth. In my experience that is done using the 8 pole "250" roofing filter (really 330), and very carefully aligning the offset in setup, so that the roofing filter skirts are centered on the DSP skirts with width at 350, MEASURING AT THE -30 dB POINTS ON BOTH SIDES. This produces a remarkably sharp, very steep skirt, that equals the remarkable selectivity of my MP with INRAD 400 and 250 8 pole filters in the 8 and 455 IF's. Of course the MP never had the IMD performance. With aligned skirts, the K3 delivers the selectivity of the MP WITHOUT the 6 dB of IMD crud down at the bottom covering up 6 dB of Russians on 40 meters. And then there's key click cancelling, see below. With the shift control set for 10 Hz increments, Mr 35over9 can be edged out with just a tick or two up or down on the shift, and STILL have enough band width to get the other 25% of the callers. You will not accomplish this on a good antenna without a roofing filter that matches your preferred operating bandwidth. For those who can't stand key clicks, this procedure has another benefit. This very sharp combination skirt turns a key click from up or down into an amplitude-reduced very sharp waveform that has a very quick onset and very quick drop out, e.g. a ** PULSE **. Because the AGC can now reach down farther, some mistakenly think this is making the click WORSE. But it's only turned the click into a REDUCED AMPLITUDE pulse. The K3 is WONDERFUL at dealing with pulses, so the DSP NB settings of T1-7 T2-7 and T3-7, IF NB **off**, with AGC set to skip pulses, kills the clicks from the guy up above or down below. The K3 is the only radio I know of that can deal with serious clicks and maintain a satisfactory run frequency bandwidth in such ghastly conditions. But you CANNOT do this just with the DSP bandwidth. You need a matching 8 pole roofing filter with offset adjusted to make roofing and DSP skirts align 30 dB down. That said, conversational use outside of contests, where you don't have super signals just up and down, you might be hard pressed to hear the pumping, even using the 5 pole standard 2.7 filter on CW. The DSP AGC will suck up a lot of the hardware variation if you are on FAST AGC and don't run PRE on and RF gain wide open all the time on low bands. 73, Guy. On Sun, Mar 25, 2012 at 8:53 AM, Don Wilhelm <w3...@embarqmail.com> wrote: > Don, > > The DSP filters can deal with all the interference as long as it is not > overloaded - and overload is what the Hardware AGC prevents. > What I am saying is - turn Hi-Cut down so you have the equal of a 1.8 or > 1.5 filter - then if you begin to hear the Hardware AGC pumping from the > nearby signals, you can conclude you need one of the narrow filters. > > The DSP filters do not need any filtering help, they are quite good all > by themselves, but observe for AGC pumping, and if present under your > contest conditions, then the only cure is a more narrow roofing filter. > > 73, > Don W3FPR > > On 3/25/2012 8:33 AM, gold...@charter.net wrote: >> After spending time in two DX contasts this year and also dealing with >> lots of splatter from other stations during non contest conditions I >> find myself continually dialing down tighter and tighter filter settings >> on phone (ssb) >> >> Currently I have a 2.7, and 2.1 filter plus the narrow ones for digital. >> >> My brain is starting to tell me to stop using the 2.1 and get either a >> 1.8 or even a 1.5 or possibly both. My thoughts are that the dsp would >> work much better if some of the interference can be directly blocked out >> by the filter. These thoughts are guided by past readings on the list >> that this is the case. >> >> What is the collective wisdom of these filter changes I am thinking >> about. I am curious about others who may have tried these and if they >> kept them or if they thought it made no difference at all. >> >> > ______________________________________________________________ > 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
