Martin, As a general rule, before adding roofing filters I'd suggest that it would be a good idea to first operate for a while with the DSP filtering at the width of the proposed roofing filter to see whether you are comfortable with that bandwidth. In the case of SSB, my understanding is that you should narrow the bandwidth by reducing the HI cut and leaving the LO cut unchanged, not by reducing the width and leaving the FC unchanged.
If you are comfortable with a narrow bandwidth but find that nearby strong signals are getting through your wider roofing filters and causing IMD or gain pumping, then you should look for the narrowest roofing filter that is the same as or wider than your preferred operating DSP bandwidth. Remember that the purpose of the roofing filters is not to set the filter bandwidth you listen with (the DSP filters do that); it is to set the bandwidth outside which strong signals will be rejected in order to give you the desired dynamic range. If you don't need a lot of dynamic range at a particular signal separation, then you don't need a roofing filter at that separation. You should also consider how you use the sub-RX before deciding whether to get two filters or only one for the main RX. In a pileup, you can use either receiver to listen to the DX and the other one to listen to the pileup (using the SPLIT button to control which VFO is used for transmit), so if you have only one roofing filter at a particular bandwidth you can decide which of the two frequencies needs it the most. Of course, if you find yourself needing the close-separation dynamic range at both frequencies, then you need both roofing filters. If you already have the 400 Hz 9-pole filters, note that the 400 Hz filter BW6 is 435 Hz and the 250 Hz BW6 is 370 Hz (look at the plots at <http://www.elecraft.com/K3/K3_8_pole_plots.htm>). That's a rather small difference, especially if you configure both roofing filters to switch in at the next DSP bandwidth below the roofing filter's actual BW6. If you really need narrower roofing filters because strong signals are getting through inside the 400 Hz roofing filter bandpass but outside the DSP filter bandpass, then I'd suggest you consider the 200 Hz filter unless you never operate with bandwidths as low as 200 Hz. The filter skirts probably do not matter, as the DSP filter skirts are much steeper anyway. If you need these roofing filters in both receivers, and if you will be using bandwidths that narrow during diversity receive, then you should probably purchase a pair of matched filters. There is no way for the user to tweak this - you can pretend the offsets are the same in the configuration, but if the hardware filter offsets are different enough, that won't work, especially at 200 Hz bandwidth where even small differences in the offset will make a large audible difference. On the other hand, if you are operating with a 200 Hz DSP filter bandwidth using a 200 Hz roofing filter in the main RX and a 400 Hz roofing filter in the sub-RX, that might be OK for diversity receive. My understanding, subject to correction as always, is that the offset problem during diversity receive is most audible with two 5-pole filters with slightly different offsets, rather than with one 5-pole and one 8-pole filter. Again, though, if you find yourself suffering from gain pumping in the sub-RX because of very strong signals outside the DSP filter but inside the sub-RX's roofing filter, that's when you will need a narrower roofing filter in the sub-RX, and in that case you will want to match its offset with the one in the main RX. 73, Rich VE3KI DM4IM wrote: > i am about to order filters for my K3 and would like to hear from the > group what filters i should go for. > I am a ragchewer as well as a contester, ssb & cw. Rarely data modes. > I do weak signal work on 160/80 cw. > I have > 2.8khz ssb in main & sub > 400hz cw in main & sub > Should i order the 200hz 5-pole filter for their narrower bandwith or > the 250hz 8-pole with steeper skirts? Add 2.1khz or 1.8khz for ssb? > What is your recommendation? > Should i add filters for both rx's (-> diversity) ? > If i go for the 5-pole filters,should they be matched by elecraft or is > there a method to tweak it in the K3 ? ______________________________________________________________ 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
