[time-nuts] Sound Card Spectrum Analyzer

Bruce Griffiths bruce.griffiths at xtra.co.nz
Fri Feb 19 01:52:59 UTC 2010


A better calibration technique may be to add a known amount of RF noise 
(eg from an amplified noise diode or similar) to one of the mixer inputs.
This allows calibration without switching the sound card preamp gain to 
a value different from that used to make measurements.
The added RF noise only has to be white over the spectrum analyser 
bandwidth.
This allows the frequency response of the entire measurement setup to be 
determined.

Bruce

Bob Camp wrote:
> Hi
>
> I did a search for *input* DC coupled sound cards and came up empty handed. There are *output* DC coupled cards out there.
>
> I have a pile of old Audigy 2's that are ripe for destruction. Essentially they are free at this point in their life. I've also got a E-MU 0202, an Xonar DX, a couple of Presonus firewire boxes running around. As the price goes up, the probability of destructive mods goes down pretty fast. The Xonar is probably the one that will be in the "final product".
>
> -----------
>
> The lock box provides an easy way to bump one source off far enough for a beat note calibration. Having a DC coupled "scope" mode would be very nice for doing an accurate calibration. Sine wave approximations are one step to many.
>
> ---------
>
> You could model the input to the sound card. I'll certainly try that on some of the more expensive boxes. Also going to 1 Hz is not an every day need. Most of these cards should do just fine at 10 Hz with no mods.
>
> --------
>
> Still looking for software.
>
> Bob
>
>
> On Feb 18, 2010, at 8:31 PM, John Miles wrote:
>
>    
>> 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
>>>>>> To unsubscribe, go to
>>>>>>              
>>>> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
>>>>          
>>>>> and follow the instructions there.
>>>>>
>>>>>            
>>>>
>>>> _______________________________________________
>>>> 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.
>>>>
>>>>
>>>> _______________________________________________
>>>> 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.
>>>
>>>        
>>
>> _______________________________________________
>> 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.
>>
>>
>> _______________________________________________
>> 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.
>>
>>      
>
> _______________________________________________
> 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.
>
>    





More information about the time-nuts mailing list