At 04:52 PM 2/14/2007, Guy Olinger, K2AV wrote: >If you didn't forget to turn your agc OFF.... > >The big unknown here would be the signal from your signal source. A >lot of signal sources to calibrate frequency and level are also not >narrow if you start looking down below. Having phasing, 120 Hz >artifacts (particularly if you powered a signal source from a wall >wart), or any other kind of noise down 30 db from the main signal is >satisfactory for many uses, but not for what you are doing, which >requires a PRISTINE signal source. > >Isn't there a way to look at a signal source with the SDR to verify >how clean a calibration signal is?
You can look at the digitized samples, but what you get is the spectrum of the source, mixed with the spectrum of the DDS, mixed with the spectrum of the A/D clock. If you're trying to measure something that's really quiet, the phase and amplitude noise of the SDR will dominate. There's an extensive literature on measuring phase noise out there.. For a narrow band source, one strategy is to mix the signal with a delayed copy of itself, and then look at the output of the mixer with a suitable narrow band filter and detector. Of course that narrow band filter and detector has it's own noise, etc. But, sometimes you can characterise the detector noise, and compare the noise power with and without the signal under test. Sort of like doing a Y factor measurement for noise figure, where you measure the output of the UUT with a load at room temperature and then with a load at a high temperature (or a calibrated noise source).. then you extrapolate back to what you would have measured without any noise contribution from the warm load. For all the hassles in improvised schemes, if you're really interested in characterizing close in phase noise, spend the $200 and get a really, really quiet XO as a comparison standard. Getting -130 dBc/Hz 10 Hz and -155 dBc/Hz at 100 Hz down to a floor of -165 dBc/Hz at 1kHz and more isn't all that tough. Spend a bit more and you can get -175 dBc/Hz at 1kHz, but the real close in is about the same. With that quiet reference, and some care and cleverness in measurement, you can characterise almost anything. Sapphire resonators are somewhat better, or you could scrounge up a Hydrogen maser. That might get you 10 dB or so better performance at 10 Hz out, say, -143 dBc/Hz, for those real eyelash of the bacteria sitting on the gnat's eyelash kind of measurements. >73, Guy. > >----- Original Message ----- >From: "Larry W8ER" <[EMAIL PROTECTED]> >To: <[EMAIL PROTECTED]> >Cc: <FlexRadio@flex-radio.biz> >Sent: Wednesday, February 14, 2007 7:22 PM >Subject: Re: [Flexradio] PowerSDR "filter" bandwidth > > > > Mark, > > > > I would assume that you also turned off your AGC. I am surprised at > > your > > figures. From what I hear, I would expect the filters to be much > > sharper > > than that. > > > > --Larry W8ER > > > > > > Mark Amos wrote: > >> Flexers, > >> > >> Has anyone done any actual measurements on filter bandwidth in > >> PowerSDR - I've been having an on-going discussion with a friend of > >> mine on filter sharpness, etc. - basically bragging about how tight > >> the software filters are. > >> > >> However, when I use a signal generator and measure the filter width > >> using the flex RX meter to measure the signals, I get much wider > >> filters than what the filter buttons say. For instance, the filter > >> bandwidths I measured come outlike this: > >> > >> 10Hz filter: 58 Hz at -3dB > >> 20Hz filter: 82 Hz at -3dB > >> 50Hz filter: 90 Hz at -3dB > >> 100Hz filter: 100 Hz at -3dB > >> > >> 10Hz filter: 260 Hz at -30dB > >> 20Hz filter: 260 Hz at -30dB > >> 50Hz filter: 280 Hz at -30dB > >> 100Hz filter: 300 Hz at -30dB > >> > >> I did these tests at 1MHz and 10MHz and got substantially similar > >> results. The 1MHz amplitude measurements ranged from roughly -40dB > >> to -90dB and the amplitudes measured at 10MHz ranged between -25dB > >> to -70dB but the shape of the curves looked about the same. > >> > >> I used a PalStar ZM-30 as a frequency generator stepping 10Hz with > >> each step (the FlexRadio corroborated the step size and the actual > >> frequency within a couple of Hz.) > >> I'm embarassed to say this is the first time I've actually measured > >> filter bandwidth.... So, I assume I'm just doing something wrong > >> and that's why the filters shapes look so fat... > >> > >> Anybody else got any empirical measurements to refute these > >> obviously flawed measurements? (Or thoughts on what I'm doing > >> wrong?) > >> > >> My qualitative assessment of the filters is that a 20Hz filter > >> "sounds" about like the 100Hz filter - so maybe it's something in > >> the way I've configured the software. Any thought? > >> > >> Mark > >> > >> > >> > >> > >> > >> -------------- next part -------------- > >> An HTML attachment was scrubbed... > >> URL: > >> > http://mail.flex-radio.biz/pipermail/flexradio_flex-radio.biz/attachments/20070214/e0d4e850/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/ > >> > >> > > > > _______________________________________________ > > 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/ > > > > > > >Scanned for viruses by Blue Coat >http://www.WinProxy.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/ James Lux, P.E. Spacecraft Radio Frequency Subsystems Group Flight Communications Systems Section Jet Propulsion Laboratory, Mail Stop 161-213 4800 Oak Grove Drive Pasadena CA 91109 tel: (818)354-2075 fax: (818)393-6875 _______________________________________________ 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/
