[time-nuts] Sub Pico Second Phase logger

[Joe Gwinn]

At 11:48 PM +0000 12/18/08, time-nuts-request at febo.com wrote:
>
>Message: 5
>Date: Fri, 19 Dec 2008 12:48:27 +1300
>From: Bruce Griffiths <bruce.griffiths at xtra.co.nz>
>Subject: Re: [time-nuts] Sub Pico Second Phase logger
>To: Discussion of precise time and frequency measurement
>	<time-nuts at febo.com>
>Message-ID: <494AE14B.2050405 at xtra.co.nz>
>Content-Type: text/plain; charset=ISO-8859-1
>
>[snip]
>
>  >>>      
>>>  This just adds another layer of complexity for little immediate gain.
>>>  Making the algorithms robust against small drifts in beat frequency is
>>>  more useful in the general case (when 2 different test sources are being
>>>  compared) than just assuming that the the beat frequency is very stable
>>>  and fixed.
>>>    
>>
>>  Yes, but I'm not sure we are solving the same problem.
>>
>>  I suppose the sound card could drive a simple PLL signal cleanup circuit.
>>
>>
>>  
>One potential problem with using a sound card for a test source is that
>the output DAC may share the same clock as the ADC ensuring that the
>output signal is locked to the ADC sampling rate.

True.  I would expect everything on a given card to use the same clock.


>In an actual dual mixer system the beat frequency and the ADC clock won't
>necessarily be synchronised (it's difficult to lock the sampling clock of
>most sound cards to an external reference).

Maybe impossible.  While stuff used by the Broadcast folk probably 
does accept external sync, it may as well be made of purest beaten 
Gold.


>If one isn't careful the algorithms developed may work well when the ADC
>sample clock and the test frequencies are locked, but have problems when
>they are not.

True.


>Using a second sound card to generate the test signal may overcome this
>problem at increased cost, and for some it may not even be an option.

It may not work with PCI soundcards, as the card clock may be 
synchronized to the PCI bus clock.  Firewire/USB cards will have 
their own independent clocks.


>[snip]
>  >>>      
>>>  The immediate task is actually to evaluate sound cards for their
>>>  suitability, preferably without the added cost and complexity of a DDS
>>>  LO and mixer.
>>>    
>>
>>  Suitability for what?  That is the point of enumerating tasks.
>>
>>  
>Suitability for use in a dual mixer system.

Ah.


>  >> 10Hz resolution whilst avoiding phase truncation spurs is impractical
>>>  with a DDS chip by itself.
>>>  Depending on the DDS and its clock frequency, the frequency spacing of
>>>  phase truncation spur free outputs may be as large as several kHz.
>>>    
>>
>>  Is this true of concatenated DDS chips?  I recall a patent to the
>  > contrary.
>  >  
>Which patent?

Hmm.  It's at work.  I'll look it up in January.  As I recall, the 
second DDS made a small integer conversion, and so had low spurs, 
while the first DDS was set to whatever was needed.


>If the zero crossings are time stamped and do not occur simultaneously
>in each channel then the phase noise of the offset oscillator will
>affect the measurement.

I'm not following.  Please expand.  The zero crossings are never 
aligned unless there is no phase delay.


>  >> A few divide and mix stages will be required to achieve a spur free
>>>  resolution of 10Hz.
>>>    
>>
>>  That is a traditional approach.  But are there alternate approaches that
>>  have now become practical?
>>
>>
>Diophantine frequency synthesis?

 From the sound of the name I think so, at least in the last DDS 
stage, as done by that patent.

But I was fishing.


>  >  
>>>  To broaden participation we need to broaden the scope of the project to
>>>  include dual mixer system with statistically independent test sources as
>>>  well as the more specialised case where the 2 input frequencies differ
>>>  only in phase.
>>>
>>>  1) Evaluate sound cards for suitablility.
>>>  Initially use simple less stable sources and follow up with more stable
>>>  test sources for the more promising cards.
>>>  Need to measure crosstalk, temporal instability of interchannel phase
>>>  shift, system noise etc.
>>>    
>>
>>  Generally agree, but there is that undefined elastic term "suitability"
>  > again.
>>
>>  
>Replace suitability with:
>Measure the characteristics of a sound card that affect the performance
>when used in a dual mixer system used to measure the relative phases of
>a pair of RF signals.
>Where the pair of RF signals may either:
>
>1) originate from 2 statistically independent sources (OCXOs, GPSDO's etc)
>
>OR
>
>2) originate from the same source and just differ in phase.

That covers it for sure.