[time-nuts] Sub Pico Second Phase logger

Bruce Griffiths bruce.griffiths at xtra.co.nz
Tue Dec 9 00:12:22 UTC 2008


Joseph M Gwinn wrote:
> Bruce,
>
>
> time-nuts-bounces at febo.com wrote on 12/08/2008 05:53:08 PM:
>
>   
>> Joseph M Gwinn wrote:
>>     
>>> People used passive mixers driving electromechanical stripchart 
>>>       
> recorders 
>   
>>> to compare high-stability oscillators in the good old days.
>>>
>>> One assumes that there is a purely analog approach to measurement of 
>>> picosecond changes in delay at 10 MHz using a single oscillator, but I 
>>>       
>
>   
>>> have not seen any methods described, probably because the relevant 
>>> articles appeared many decades ago.
>>>
>>> Can anyone suggest some articles to read?
>>>
>>> Thanks,
>>>
>>> Joe Gwinn
>>>
>>>       
>> Joe
>>
>> Although one could in principle do this with a single diode double
>> balanced mixer used as a phase detector all one may end up measuring is
>> the effect of ambient temperature changes on the mixer phase shift.
>> Lower mixer phase shift tempcos are possible if the RF port is 
>> unsaturated.
>>     
>
> Single diode?  Why wouldn't one use a standard (MiniCircuits or the like) 
> four-diode two-transformer double-balanced mixer as the phase detector? 
> Many mixers have IF response down to DC.
>
>   
Oops, I meant "single diode type double balanced mixer style phase
detector".
>   
>> A classical dual mixer system is probably better in that with matched
>> tempco mixers maintained at the same temperature the differential phase
>> shift tempco should (with careful matching) be lower.
>>     
>
> Dual mixer as in DMTD (dual mixer time difference) would certainly work, 
> but is pretty complex and temperature sensitive.
>
> I did use a loaner Symmetricom 5120A (a full digital DMTD implementation) 
> to make some measurements six months ago, and after a few days of 
> continuous operation it had settled to the point that one could see 0.01 
> pS changes.  (And touching one of the BNC connectors caused a 1-3 pS 
> jump.)  This instrument costs about $30K, and is intended more for 
> measuring phase noise and allan variance than delay changes.
>
> Anyway, I have to wonder what people did before DMTD was invented.
>
>
>   
>> Other than the numerous classical papers on dual mixer systems and the
>> occasionl NIST paper that have some mixer phase shift tempco data
>> (albeit sparse), I am not aware of any specific papers.
>>     
>
> I've read many or most of the classical DMTD papers, and have seen various 
> passing estimates that diode-ring mixers have a temperature sensitivity of 
> 8 to 10 pS per degree C.  (I recall your figure was 10 pS/K.)  I assume 
> that the DC offset also varies with themerature and drive signal 
> amplitude.
>   
The only reference I have on the offset tempco is a miniciruits
application note from which one can deduce that the equivalent phase
shift tempco associated with the offset tempco is a few hundred
femtosec/C (@ 10MHz +7dBm) at some temperatures for the particular mixer
used. The graph also indicated (if you are lucky) that the offset tempco
may be zero at around 20C.
A NIST paper indicated that mixer phase shift tempco was around 10x
lower if the Rf port was unsaturated. It also indicated that the mixer
phase shift tempco is much lower if the input frequency is 100MHz rather
than 10MHz. This was one reason given for shifting to 100MHz DMTD systems.
>  
>   
>> A purely analog approach to phase shift measurement has to be more
>> difficult than a hybrid one using a pair of low frequency ADCs (eg high
>> end sound card).
>>     
>
> Is the sound-card approach workable at the millidegree to microdegree 
> level, if the change is spread out over an hour?  One picosecond at 10 MHz 
> is 3.6 millidegrees of phase.
>
> Joe
>
>   
Preliminary (non optimum) tests by Ulrich indicate that picosecond
stability for times up to 100sec is very easy to achieve.
Beyond that mixer phase shift tempco mismatch may be significant.
ADEV noise level of around 2E-14/Tau (1s < tau <100s).
Haven't yet seen have data for longer tau.
With identical beat frequency outputs crosstalk between channels within
the sound card shouldn't be a great problem.
In any case its very easy to measure the crosstalk transfer function.

One concern particularly for low beat frequencies is the phase shift in
the sound card input coupling capacitors (usually electrolytics).

It should be easy to test the sound card phase shift stability for this
application by driving both inputs from the same signal source.

Bruce




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