Bob, Thanks!
I typically use 48000 sampling with the FA-66, mostly for the sharpest filters with CW. In any case, I don't have a need for an IF above 20Khz -just interested in learning! The VB version was before my time so thanks for the history lesson! Mark -----Original Message----- From: Robert McGwier [mailto:[EMAIL PROTECTED] Sent: Sunday, May 06, 2007 11:20 AM To: Mark Amos Cc: flexradio@flex-radio.biz Subject: Re: [Flexradio] Dual watch with SDR-1000... Mark Amos wrote: > Bob, > > Thanks for that info! I hadn't noticed that control before. It's nice to > have the flexibility to reduce this asymmetry by lowering the IF and it's > also nice to move the DC noise hump more out of the way by increasing it. > > I see that the IF selection upper limit is 20000. Can you tell me what the > reason is for this limit (assuming that it's not just > "why-would-anyone-ever-want-an-IF-higher-than-20000".) That is also legacy. With DttSP and PowerSDR, 44100 was never supported so far as I can remember. I do not recall a version of DttSP which ever allowed 44100 as a sample rate. We started with, and have always had 48000 as a lower limit is my recollection. 44100 was there in the Visual Basic 6 console and it had NO variable base IF. It was always 11025 Hz +/- spur reduction offset. First, on 48000 Hz sample rate, and assuming you are in USB, you probably want at least 3500 Hz for the USB filter. Nyquist is 24000 Hz as you have pointed out. 24000 - 3500 Hz bandwidth is 21500 Hz. AM and FM want wider bandwidths, etc. The number makes sense for all sorts of reasons when the sample rate is 48000 Hz. It simply was never changed when I added the higher rates. We are still living with the consequences of me getting a wild hair one night and throwing 96000 and 192000 Hz sample rate support in. It has taken Eric months to wrestle all of the consequences to the ground. This is another example. Bob N4HY > > I thought it was the case that the original IF was put there to keep the sum > of the sampling rates of I and Q under the N/2 limit for 44100 sampling? If > so, isn't this cutting it a bit close? (i.e. isn't it typical to keep the > sampling somewhat under .5N so that anti-alias filters can be a little less > demanding to design.) > > Just trying to learn more from the discussion. > > Mark > > -- AMSAT Director and VP Engineering. Member: ARRL, AMSAT-DL, TAPR, Packrats, NJQRP, QRP ARCI, QCWA, FRC. ARRL SDR WG Chair "If you're going to be crazy, you have to get paid for it or else you're going to be locked up." Hunter S. Thompson _______________________________________________ FlexRadio mailing list FlexRadio@flex-radio.biz http://mail.flex-radio.biz/mailman/listinfo/flexradio_flex-radio.biz Archive Link: http://www.mail-archive.com/flexradio%40flex-radio.biz/ FlexRadio Knowledge Base: http://kb.flex-radio.com/ FlexRadio Homepage: http://www.flex-radio.com/
