Again and again, we all encounter signal conditions where a very weak wanted signal is close to a very loud unwanted signal. For this the best filter setup is where BOTH the roofing and the DSP filters are at the desired operating bandwidth.
That is why for CW contesting, in pure self-defense, the 400 and 250 8 pole roofing filters are defined as 450 and 350 respectively. 450 is a "running" bandwidth for 500 Hz separation that recognizes that many answerers to our CQ's do not have the ears for getting close to frequency. When up and down loud signals get too close, dropping to 350 Hz width and using shift will help enormously without losing too much of the space for up and down callers. Additionally, I make sure that the filter offsets are set so the minus 30 dB points of the two filters coincide. The desired effect is that when you tune away from an unwanted signal, at the passband edge it falls off like a ball slowly rolled off a table. This procedure finally made the K3 selectivity as sharp as my MP with the Inrad 8 and 455 filters. One does NOT have to treat the roofing filter as if it is not involved in selectivity. Quite the contrary, as when a certain 40m Italian station is 35 over 9 up 350 Hz and I am being called by a QRP station using a wet noodle antenna on his basement floor, who is well down in the S2 noise, who also insists on adding /QRP to the end of his call. This becomes more and more important as these RX improvements keep shrinking what we thought was incoming noise and we hear ever weaker signals. Turning the edge of roofer plus DSP into a knife is VERY useful. 73, Guy K2AV On Monday, September 28, 2015, Bob McGraw - K4TAX <rmcg...@blomand.net> wrote: > Ken makes a pertinent comment; "the DSP does all the rest". Suggested > filters are roofing filters and serve a function of signal path and how > signals outside of the DSP bandwidth can affect the receive performance. > Receiver selectivity is actually accomplished by the DSP engine. > -- Sent via Gmail Mobile on my iPhone ______________________________________________________________ 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 Message delivered to arch...@mail-archive.com
