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/
