Got it. So you're planning to have a way to supply an offset frequency to generate a beatnote for calibration at some known reduced amplitude, correct? That, and an FFT program that understands how to normalize to 1 Hz BW (and correct the window's noise-equivalent bandwidth) should be all you need. Bonus if the FFT program allows the calibration factor to be added to whatever numbers it draws on the Y-axis.
What brand/model sound card are you looking to use? With a white-noise source you might be able to add a calibration step that takes the effect of the input coupling capacitor out of the measurement, so you wouldn't need to modify the card itself. There was an article in QEX a year or so ago on the basic idea. Again, this all assumes you have some control over the software. There may also be some DC-coupled sound cards at that end of the market, too -- I haven't looked lately. -- john, KE5FX > -----Original Message----- > From: [email protected] [mailto:[email protected]]on > Behalf Of Bob Camp > Sent: Thursday, February 18, 2010 5:10 PM > To: Discussion of precise time and frequency measurement > Subject: Re: [time-nuts] Sound Card Spectrum Analyzer > > > Hi > > Ok, A bit more info: > > 1) Quadrature PLL using an RPD-1 DBM and a home brew lock box. > 2) Willingness to accept that I'm measuring a pair of oscillators > 3) Plenty of sources at the appropriate frequencies > 4) First took a shot at this in 1975 (I forget the Fluke app note > number ...) been doing it ever since > 5) Appropriate preamp between the RPD and sound card is a work in progress > 6) Sound card is a 192 KHz / 24 bit / ~110 db snr class card > 7) Sound card will get butchered for the application. > 8) Should be able to hit -165 ish floor, -120 ish at 1 Hz > > Except for the 16 bit limitation, Baudline looks like it's got > what I need. > > Bob > > > On Feb 18, 2010, at 7:53 PM, John Miles wrote: > > > Unfortunately there's no way to build a sound-card application that can > > measure phase noise in the general case without a lot of additional > > hardware. Baseband PN analysis with an FFT presupposes that > you have some > > external means of downconverting the DUT signal to DC with a superior > > reference at the same frequency, tuned with a quadrature PLL. > There must be > > a suitable high-pass filter and LNA to block any DC residuals > and preamplify > > the remaining part of the noise sideband. > > > > Further, it's often the case that noise close to the carrier is strong > > enough to keep you from being able to use enough gain to see > the broadband > > floor, so you actually need more than one high-pass filter ahead of the > > sound card in many cases. These switchable filters were > mandatory with the > > old 13-bit signal analyzers like the 3561A, and may still be > needed today if > > you want to look down to 1 Hz. If you restrict your offset > range to (say) > > 100 Hz to 20 kHz and require a 24-bit sound card, you can > probably get away > > without the switchable HPFs. > > > > It'd be helpful to know exactly what sorts of measurements you > need to make, > > and on what devices. PN measurement is a *lot* of work, on both the > > software and hardware sides. Much of it goes into developing a suitable > > calibration process. Take a look at the 3048A manuals > sometime, realizing > > that the 3048A hardware itself is not very complicated... > > > > -- john, KE5FX > > > >> -----Original Message----- > >> From: [email protected] [mailto:[email protected]]on > >> Behalf Of Bob Camp > >> Sent: Thursday, February 18, 2010 4:18 PM > >> To: Discussion of precise time and frequency measurement > >> Subject: Re: [time-nuts] Sound Card Spectrum Analyzer > >> > >> > >> Hi > >> > >> Both are very cool programs. Both are full of all sorts of neat > >> features. As far as I can see, neither one has the features I'm after. > >> > >> More or less - I want it to run like a clunky HP audio analyzer > >> rather than a very cool tool for ham radio. > >> > >> Bob > >> > >> On Feb 18, 2010, at 6:33 PM, Don Latham wrote: > >> > >>> You have looked at: > >>> spectran and spectrum lab ? > >>> Don > >>> > >>> Bob Camp > >>>> Hi > >>>> > >>>> Assuming I have a decent sound card, and a computer, the next > >> thing I need > >>>> is software. If I want: > >>>> > >>>> Required: > >>>> > >>>> 1) non- commercial > >>>> 2) 1 Hz normalization > >>>> 3) good low frequency processing (decimation ahead of the fft) > >>>> 4) low cost > >>>> > >>>> Much preferred: > >>>> > >>>> 5) a non-evil OS > >>>> 6) Rational performance on a non-quad core system > >>>> 7) free > >>>> 8) rational calibration > >>>> 9) scope view. > >>>> 10) reasonable graphics > >>>> 11) active support by the author > >>>> > >>>> The application is measuring phase noise. That what makes 2 & > >> 3 pop up on > >>>> the list. > >>>> > >>>> I've looked at a lot of programs and they all seem to be > >> pretty slick. The > >>>> ones I've looked at so far don't quite hit the mark for phase > >> noise. I'm > >>>> pretty sure that there are others on the list who have dug > >> into this same > >>>> issue already. > >>>> > >>>> Bob > >>>> _______________________________________________ > >>>> time-nuts mailing list -- [email protected] > >>>> To unsubscribe, go to > >>>> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > >>>> and follow the instructions there. > >>>> > >>> > >>> > >>> -- > >>> Dr. Don Latham AJ7LL > >>> Six Mile Systems LLP > >>> 17850 Six Mile Road > >>> POB 134 > >>> Huson, MT, 59846 > >>> VOX 406-626-4304 > >>> www.lightningforensics.com > >>> www.sixmilesystems.com > >>> > >>> > >>> _______________________________________________ > >>> time-nuts mailing list -- [email protected] > >>> To unsubscribe, go to > > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > >> and follow the instructions there. > >> > > > > > > _______________________________________________ > > time-nuts mailing list -- [email protected] > > To unsubscribe, go to > > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > > and follow the instructions there. > > > > > > _______________________________________________ > > time-nuts mailing list -- [email protected] > > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. > _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
