[time-nuts] another Ebay mixup, 5370

Dr Bruce Griffiths bruce.griffiths at xtra.co.nz
Sun Jun 10 09:32:51 EDT 2007


Tom Van Baak wrote:
>> In the case of the 5370, page 3-12 of the manual is where it came from ("15.
>> Press STD DEV and +/- TI switch.  Display should read less than 100 ps (this
>> reading is the instrument's jitter).")
> I'm wondering if it would be more informative to make three
> runs: one with A and B uncorrelated to ref; one with A=ref
> and one with B=ref.
>> At some point between my original 2316 edition and the 2904 edition scanned
>> by David Kirkby, they apparently changed this step to call for pressing the
>> +TI switch, unless the +/- symbol failed to scan properly.  I get about
>> 10-15 ps more jitter in +TI ONLY mode for whatever reason.
> Agreed, that's odd. Bruce, any ideas on this one?
Whenever one uses an asynchronous input frequency, the start and stop 
interpolators are used over their full range (or a substantial part 
thereof) so that the variations in the difference in differential 
nonlinearities of the 2 interpolators is combined with the intrinsic 
jitter when calculating the jitter from the series of period measurements.
When using +TI only the START event precedes the STOP event and the 
period of the input waveform is measured (the differential delay of the 
START and STOP channels as well as the effect of any START and STOP 
trigger mismatch affect the measurement).

When using +-TI then the START and STOP events occur on the same edge of 
the input waveform so that the measured time interval is nominally zero.
When the START and STOP events are separated by 1 period of the input 
signal (100ns for a 10MHz input) the difference in the differential non 
linearities of the START and STOP interpolators are not as well matched 
than when the the START and STOP events are nominally coincident. Thus 
the calculated jitter will be larger for the +TI only setting.

The jitter measurement with an asynchronous input signal is more 
representative of the result when measuring signals that are not 
coherent with the internal timebase.
When using the internal timebase as the input signal the START and STOP 
events will tend to exercise the same narrow region of the interpolators 
so that the calculated jitter will be dominated by noise rather than the 
interpolators differential non linearity characteristics.
>> The last time I studied the manual, I remember being convinced that the 5370
>> would be relatively immune to clock-correlation effects if the interpolators
>> were set up properly.
Wishful thinking, the designers don't believe this so why should we?

I have a copy of a letter from David Chu detailing the then known 
sources of differential nonlinearity in the 5370A.
These causes are principally crosstalk between the START and STOP 
channels, and interaction between the mixers and the phase locked 
> Right, but that's a big "if", especially if the unit is surplus. So it's
> best to assume the interpolators aren't perfectly tuned which is
> why keeping ref away from the A or B inputs is important.
> /tvb

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