[time-nuts] Sub Pico Second Phase logger

Joe Gwinn joegwinn at comcast.net
Fri Dec 19 17:57:35 UTC 2008


Bruce,

At 3:54 AM +0000 12/19/08, time-nuts-request at febo.com wrote:
>
>Message: 5
>Date: Fri, 19 Dec 2008 16:51:55 +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>
>
>Joe
>
>Joe Gwinn wrote:
>>  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>
>  >>
>  >> [snip]
>  >  
>>>  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.
>>
>>  
>Most recent design PCI sound cards have their own independent crystal
>oscillator.

It won't be a great crystal, but a crystal nonetheless.


>Some claim to be able to sync to an SPDIF input but the resultant jitter
>may be large.

Why large jitter?  Bad implementation?


>  >  
>>>   >> 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.
>>
>>
>Do you mean US5598440?

Yes, that's it.


>  >> 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.
>>
>>
>>  
>Yes, thats the point, the offset generator phase noise contribution isnt
>the same for both zero crossings.
>Greenhall et al correct for this to some extent, but at least for short
>tau, one is then no longer measuring ADEV, MDEV etc.
>There is some advantage in having a higher beat frequency as the offset
>generator phase noise has less time to accumulate.

I recall reading an article on this by Greenhall, probably _the_ 
article.  My reaction was that I would be fortunate to have a setup 
where I could even detect such a problem.


>  >>  >> 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.
>>
>>  
>Conventional Diophantine synthesis uses number theory together with 2 or
>3 conventional synthesiser loops to achieve very fine resolution whilst
>maintaining a high PLL phase detector input frequency.

In a sense, the concatenated DDS approach is a divide-and-mix chain. 
Perhaps there is a parallel here.




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