[time-nuts] T.I. experimenting - newbie question

Bruce Griffiths bruce.griffiths at xtra.co.nz
Sun Apr 3 02:03:37 UTC 2011


Bruce Griffiths wrote:
> Joseph Gray wrote:
>> I have a borrowed HP 5328A with the Universal Module. I'm using
>> averaging mode and auto triggering. Counter does not have the OCXO
>> option. I'm using the internal oscillator, not locking to my GPSDO.
>>
>> I was just messing around with a length of Belden 9273 and the 100 KHz
>> square wave output from my Spectracom distribution amp. Start signal
>> is the 100 KHz signal. Using a tee, the stop signal is the same 100
>> KHz after passing through the length of coax. I'm reading 18.9 ns. The
>> coax specs give a VOP of 66%. This should calculate out to a length of
>> 3.74 M. When I measure the coax, I get 3.8 M.
>>
>> I am including the BNC connectors in my length measurement. If I
>> subtract them, I'm much closer to my calculated length. Should the BNC
>> connector lengths be included or not? Or, is the measurement I'm
>> getting as good as it gets with this HP counter?
>>
>> Joe Gray
>> W5JG
>>
> The measured delay includes the time offset between the start and stop 
> inputs.
> This is unlikely to be zero, you need to measure it and subtract the 
> offset from the measured result.
> Since the 5328A can only measure positive time intervals measuring 
> this offset is somewhat tricky to do.
>
> Bruce
>
The simplest way of measuring the cable delay with the 5328A is to 
measure the delay with a short length of cable between the start and 
stop inputs and then increase the length of cable by connecting the 
cable to be measured in series with the first cable measure the delay 
again and subtract the first measurement from the second.
This procedure corrects for the start to stop offset as well as that of 
the first cable.
Then the effects of finite rise time, trigger thresholds and reflections 
should be considered.
One then has to consider the effects of the various connectors,





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