Mark and Bill, I made some measurements and got similar results as Mark. The one thing that confused me was the difference in shape factor between the narrow filters and the wide filters, but I think you just cleared that up for me, Bill. It has to be a function of the bin resolution and the bin bleed. Thanks, Mark for bringing this up, and thanks Bill for clearing my confusion...very interesting.
Tom W0IVJ Bill Tracey wrote: >I'm not a dsp guru, but I think what you're seeing at the smaller filter >sizes is an artifact of spectrum leakage >(http://www.dsptutor.freeuk.com/analyser/guidance.html#leakage) from one >bin to another. The basic FFT bin size is 11hz, so for a filter 30 hz or >below you've only got 3 bins to work with so I think the effects of leakage >will be relatively larger than with larger filters since the transition >zones cover more bins. > >I'd think to improve this one would need to band pass filter the signal of >interest, then decimate and do an FFT of the decimated signal such that >you have a smaller bin width. Might be an interesting hack to try when >the passband is small. > >Regards, > >Bill (kd5tfd) > > >At 07:01 AM 2/18/2007, Mark Amos wrote: > > >>All >>Here are some filter measurements I did with the Flex-Radio. I'd be >>interested in anyone else's numbers if there are differences - there may >>be some configuration or setup option >> >> ><....> > > > >>Also, I am interested in knowing if the shape factor is designed to be >>wider with narrow filters and get tighter as the filter bandwidth >>increases (or if this is an artifact of my measurements, physics, etc.) >>I've seen that steep skirts on analog filters cause ringing, but that this >>can be avoided with DSP filters. If this is the case, why not use >>arbitrarily steep filters? Is it a computational cost issue, or are there >>other tradeoffs that make this impractical. This isn't a criticism; I'd >>just like to know how it works. >>Thanks again to all the suggestions and discussion - I've learned a lot >> >> >>from you guys, and I really appreciate it! > > >>Mark >> >> > ><....> > > > >_______________________________________________ >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 Homepage: http://www.flex-radio.com/ > >FlexRadio Knowledge Base: http://kb.flex-radio.com/ > > > > > -------------- next part -------------- An HTML attachment was scrubbed... URL: http://mail.flex-radio.biz/pipermail/flexradio_flex-radio.biz/attachments/20070218/19eea4ab/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 Homepage: http://www.flex-radio.com/ FlexRadio Knowledge Base: http://kb.flex-radio.com/
