[time-nuts] Sound Card Spectrum Analyzer

Bob Camp lists at cq.nu
Fri Feb 19 01:10:22 UTC 2010


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
>>>> _______________________________________________
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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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