[time-nuts] Sound Card Spectrum Analyzer

Bob Camp lists at cq.nu
Fri Feb 19 01:33:51 UTC 2010


Hi

It's certainly got all the blocks you would need to do the processing. Linking it to a decent interface looks like a bit of a chore. I plead "LAZY" when it comes to doing this from scratch. 

I keep thinking there's got to be something out there that does all this ....

Bob


On Feb 18, 2010, at 8:27 PM, John Ackermann N8UR wrote:

> You could also consider whether Gnuradio (http://www.gnuradio.org) could do what you need.  It has interfaces to the Linux sound system.
> 
> John
> ----
> 
> Bob Camp said the following on 02/18/2010 08:10 PM:
>> 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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