[time-nuts] Sound Card Spectrum Analyzer

John Miles jmiles at pop.net
Fri Feb 19 01:31:53 UTC 2010


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: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com]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: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com]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 -- time-nuts at febo.com
> >>>> 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 -- time-nuts at febo.com
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> >> and follow the instructions there.
> >>
> >
> >
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