[time-nuts] Looking for Wavecrest Visi

John Ackermann N8UR jra at febo.com
Mon Apr 9 12:40:50 UTC 2007


Dr Bruce Griffiths said the following on 04/08/2007 10:01 PM:

>> By doing this, you can actually observe the measured value increase very  
>> slowly, one can even observe the sub 1ps values increase! Doing this, you can  
>> see about 3ps of added delay for every single turn of the SMA connector ground  
>> nut.
>>  
>> Not sure many other instruments can do that.
>>  
>>   
> There's no particular reason that they cannot if they have adequate 
> resolution and stability and a sufficient number of measurements are 
> averaged.
> This should be possible even with an HP5370A/B albeit with a slower 
> response time.

I routinely measure coax cable lengths to 200 femtosecond resolution
with my 5370B (though to be practical I normally round to at least 10
and sometimes 100 picoseconds, to take into account possible measurement
errors like adapter lengths and cable tempco).

I recently built 6 nominally 10 foot GPS antenna cables out of LMR-400.
 They all had N connectors on one end, but the opposite ends were two
each of N, BNC, and TNC, which made measurement interesting because the
needed tweenie configurations and lengths were different.

After doing the best I could to normalize out the adapters, I measured
the six cables and the spread from longest to shortest was 61
picoseconds (nominal length was 12.55 nanoseconds); I suspect that as
much as 40 ps of that was due to uncalibrated tweenie lengths because
the three pairs of cables with identical connectors matched within 15,
9, and 20 picoseconds.  (And the coax was measured and cut by hand, so
there was certainly some room for error there!)

The technique was to feed a 1000 Hz pulse train (generated from a 5369A
time synthesizer, but any good pulse generator would do) into the 5370B
TIC by routing the pulses into a T connector connected to the counter
START input; the other side of the T went to a four foot cable to
provide some additional delay.

The cable under test was connected to that cable, then to another four
foot cable, and then to the stop input of the counter.  I first measured
the delay with just the two fixture cables and N adapters in place,
which as 12.579 nanoseconds; I then subtracted the estimated length of
the N barrel connector that substituted for the cable under test.  Then
I added the cable under test, remeasured, and subtracted out the fixture
delay to get the cable length.

I used a 100k sample average which yields (IIRC) 200 femtosecond
resolution in the 5370B, and for each run checked the min, max, and
standard deviation statistics to make sure nothing goofy was going on
Standard deviation for a run like that on my 5370B is typically 30 to 40ps.

John




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