[time-nuts] Sound Card Spectrum Analyzer

[John Miles]

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
> >> _______________________________________________
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> >
> >
> > --
> > 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
> >
> >
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>


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