Mark, Nice piece of work. This series of measurements is a good illustration of the effects of the size of the DSP buffer. Did you run the test with the DSP buffer set at 4096? I always run my DSP buffer at 4096, and get basically the result that the 25 Hz filter is 25 Hz at -6dB with a 48Ks/s rate. I get results similar to yours running my FireBox at 96000s/s with the 4096 Buffer or when using the 2048 DSP buffer and sampling at 48Ks/s, but a little wider. My results show the 25 Hz filter to be approximately 50 Hz wide when sampling at 48Ks/s and using a 2048 DSP buffer.
The math of the shape 6/60 dB shape factor will normally cause the wider filter to have a smaller shape factor, with the same filter roll off. You might want to look at the enhancement list. I submitted an enhancement this morning to add 8192 and 16348 DSP buffers to improve the filter shape factor when sampling at 96K and 192K s/s. At 08:01 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 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 > >xRadio Knowledge Base: http://kb.flex-radio.com/ _______________________________________________ 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/
