We use Blackman-Harris windows for all the right reasons as the window in the filter design. We care about the ultimate out of band rejection, and this can only be accomplished by a (VERY SMALL) sacrifice in shape factor. These filters in their worst case, 192000 samples per second and DSP set to 512 sample buffers, BETTER than can be had with any traditional analog filtering system.
This is easy for you to do yourself. Set display to spectrum. Set filter size. Go to Setup panel and test tab and turn on the noise generator and select input. This will run white noise into the receiver and you can measure the filter shape DIRECTLY and capture screen shots to demonstrate the filter shape. Frankly, we are just too covered up with other work to do this now. But it is easy to do. Bob Frank Brickle wrote: > The problem with measuring the shape factor is that it can be changed > significantly by applying a different window in the filter design code. The > measured shape is a snapshot of one possible configuration. It's a tunable > parameter, however, even if it hasn't been brought out to the "front panel" > yet. > > 73 > Frank > AB2KT > > On 4/28/07, Tim Ellison <[EMAIL PROTECTED]> wrote: >> I have a grasp on the math, I an need some "real" numbers (measured) to >> fill in a product comparison of the Flex-500 vs.. it's competition. I found >> some references in a few e-mail that are about 1.5 years old. >> >> -Tim >> >> ----- >> >> Integrated Technical Services >> >> >> ------------------------------ >> *From:* [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] *On >> Behalf Of *Frank Brickle >> *Sent:* Saturday, April 28, 2007 12:40 PM >> *To:* Tim Ellison >> *Cc:* flexradio@flex-radio.biz >> *Subject:* Re: [Flexradio] PowerSDR filter shape factor? >> >> On 4/28/07, Tim Ellison <[EMAIL PROTECTED]> wrote: >>> On the web page, it states that the filter shape factor for a 500 Hz >>> filter is 1.35:1 and a 2.6 KHz filter is 1.06:1. But I believe that >>> these values were taken at 48K sampling rate with a 1024 DSP filter. >>> >>> I don't have a scope or a signal generator to check this, so I am asking >>> >>> if anyone has done (or would consider doing) an analysis of the filter >>> shape factor with different DSP buffer sizes and sampling rates? I >>> would be very interested in seeing how the shape factor varies with >>> these parameters at a 500 Hz and 2.6 KHz filter width? >> >> >> The shape factor is proportional to the sample rate divided by the filter >> length. If you double the sample rate but keep the filter length the same, >> the shape factor goes up. If you keep the sample rate the same but double >> the filter length, the shape factor goes down. The reason I only say >> proportional is that what changes the most in this picture is the transition >> bandwidth. >> >> The back of the envelope here says, at 48kHz, difference between 1.35:1 at >> 500Hz and 1.06:1 at 2.6kHz is about 30Hz in the transition bands at -60dB. >> Not much. >> >> The rule of thumb is, for a given shape factor, if you double the sample >> rate, you need to double the filter length to keep the same shape factor. >> >> 73 >> Frank >> AB2KT >> >> >> >> > -------------- next part -------------- > An HTML attachment was scrubbed... > URL: > http://mail.flex-radio.biz/pipermail/flexradio_flex-radio.biz/attachments/20070428/d07582fa/attachment.html > > _______________________________________________ > 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/ > > -- 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/
