> 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!
That's not true. Even the Flex will not be able to remove a -50 dBm signal (S9 +20) and allow you to copy a -135 dBm signal (at the noise floor) 50 or 100 Hz away. That's a blocking issue not an IMD issue. True IMD requires two or more interfering signals with a specific relationship and with narrow filters one of the two tones is almost never inside the filter. 73, ... Joe, W4TV > -----Original Message----- > From: elecraft-boun...@mailman.qth.net > [mailto:elecraft-boun...@mailman.qth.net] On Behalf Of William Carver > Sent: Friday, August 07, 2009 3:16 AM > To: Elecraft@mailman.qth.net > Subject: [Elecraft] Roofing filter > > > 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 ______________________________________________________________ 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
