It's not true that once signals get through the xtal filter they cannot be removed. A DSP filter CAN remove them...after all, the FLEX radio works without ANY roofing filter! But DSP can only do it when fed with undistorted signals within the range of its A to D converter. The circuitry following the filter needs to have very low distortion.
The K3 has more than 100 dB of dynamic range at the first mixer, say for signals spaced 10 KHz apart. What if two signals are 500 Hz apart, both passing through the 2.8 KHz filter? Can we expect a 50 Hz wide DSP filter to filter those two tones out? I expect it. My Orion-one was fitted with a 1 KHz filter and it worked very well on CW. I have a homebrew receiver with a 3.1 KHz wide filter and it worked well on CW. 'Course in both cases, no matter how good without, they'd be better if preceded by a CW-width xtal filter. And at those spacings the LO phase noise has to be lower for the desired weak signal to be heard. Maybe we're talking K4 here. Bill - W7AAZ P.S. Had a nice chat with a guy in Atlanta tonight, first QSO of newborn K3. (with, I must add, 2.8, 0.4 and 0.25 KHz 8 pile filters!) ______________________________________________________________ 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
