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 that I've overlooked that would affect the results. FYI, I'm interested in cases where someone has come up with different results and has some ideas as to why they're different (as opposed to theoretical or hypothetical suggestions without any any substantiating measurements...) I should have been from Missouri. 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 Here are (I think) the salient features of my setup and the test conditions: - PowerSDR v1.9.0 SVN 899 - Edirol FA-66 - 48000 Ks/s - Buffer 2048 - 4096 FFT Bins - Hanning window - AGC off (it didn't seem to make any difference - I assume the S-Meter is before the AGC.) I used an HP 10811 OCXO as the oscillator at 10MHz because it's the cleanest, most stable oscillator I have. Before the test I ran the receive image rejection calibration and calibrated the S-meter with a 50uV and a 1uV signal to get it as accurate as possible. I tuned the receiver up and down in 1 Hz increments and measured the signal level as reported by the Flex S-meter - to the nearest 10th of a dBm. When I finished the series, I repeated the 10Hz and 20Hz filter tests just to see if anything had drifted or changed. The results were the same.
Here are the results: 10Hz filter -6dB 32Hz wide -60dB 89Hz wide 2.78 shape factor 20Hz filter -6dB 34Hz wide -60dB 95Hz wide 2.76 shape factor 30Hz filter -6dB 38Hz wide -60dB 102Hz wide 2.68 shape factor 50Hz filter -6dB 51Hz wide -60dB 122Hz wide 2.39 shape factor 100Hz filter -6dB 100Hz wide -60dB 172Hz wide 1.72 shape factor 200Hz filter -6dB 200Hz wide -60dB 272Hz wide 1.36 shape factor 250Hz filter -6dB 250Hz wide -60dB 322Hz wide 1.29 shape factor 500Hz filter -6dB 500Hz wide -60dB 574Hz wide 1.15 shape factor 600Hz filter -6dB 600Hz wide -60dB 674Hz wide 1.12 shape factor 1000Hz filter -6dB 1000Hz wide -60dB 1074Hz wide 1.07 shape factor 2400Hz filter -6dB 2400Hz wide -60dB 2474Hz wide 1.03 shape factor So the filter algorithm is more accurate above 30Hz or so. I suspect that there are rounding errors and other issues that keep them from measuring accurately down below 30Hz. Mark -------------- next part -------------- An HTML attachment was scrubbed... URL: http://mail.flex-radio.biz/pipermail/flexradio_flex-radio.biz/attachments/20070218/8fc5a6c0/attachment.html _______________________________________________ FlexRadio mailing list [email protected] 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/
