[time-nuts] PRS-10 findings

Dr Bruce Griffiths bruce.griffiths at xtra.co.nz
Wed May 30 10:27:09 EDT 2007


Poul-Henning Kamp wrote:
> In message <v04210100c2831acb5ca5@[10.0.0.3]>, Peter Vince writes:
>
>   
>> 	I have seen something similar with my 53132A.  I was checking 
>> on the delay variation of an amplifier distributing 10 MHz, and 
>> noticed a regular sinusoidal pattern, about a third of a nanosecond 
>> peak-to-peak, with a period of about 70 seconds.
>>     
>
>   
A manifestation of the effects of slowly sweeping through a range of 
start and stop interpolator values so that the interpolator integral and 
and differential non linearities become apparent?
Strictly the difference between the nonlinearities of the start and stop 
interpolators becomes visible.
The resultant variation of around 300ps pp are well within the 
specifications for the counter.
> Almost any kind of interference will cause such anomalies and the
> closer the frequencies are to a multiple of each other, the longer
> the period will be.
>
> It is quite common for the frequency difference between the counters
> internal X-tal and the measued frequency to show up like that once
> you start to measure down in the nanosecond end of things.
>
> The HP5370 has a rather heavyhanded piece of electronics that
> eliminate this effect with a jitter based approach and as far as I
> have been able to measure, it works.
>
>   
Not quite true the HP5370 has a whole host of anomalies like 
differential linearity errors of 100psec or more for certain time 
interval ranges, at least according to its designers.
The identified causes are:
Crosstalk between microstriplines used for each channel (effective only 
when the affected signals are simultaneously active near a trigger 
point) and modulation of the internal 200MHz reference by the mixer 
outputs (always present with a quasi period of ~5.02ns).

> "normal" counters don't have this, as they are not designed to measure
> in that domain of disturbances.
>
> The easiest way to determine if this is indeed the problem, is to
> feed the counter an external frequency which can be varied a bit
> up and down.  If the period of the artifact changes accordingly: QED.
>   
Bruce




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