[time-nuts] Basic TIC measurement

Bruce Griffiths bruce.griffiths at xtra.co.nz
Wed Oct 7 04:38:23 UTC 2009


Joseph Gray wrote:
> Bruce, thanks for the very helpful advice. My comments are interspersed below.
>> You need to measure this delay.
>> Split the signal in 2 and feed it to both inputs with nominally equal
>> length cables.
>> Measure the difference then swap the 2 cables over and measure the
>> difference.
>> Average the 2 differences to get the interchannel differential delay.
>> This fails if the counter cant measure negative delays and a more
>> complex procedure is required.
> I get 0.9nS one way and 1.1nS with cables swapped. This with tees and
> terminators on each input. The cables are almost exactly the same
> length. So, I have a 1nS difference between the two inputs.
Did you use a resistive splitter or just a tee to split the signal?
>> Another issue is if the slew rate of the signal at the trigger threshold
>> is low then trigger threshold offsets between the 2 counter inputs will
>> affect the effective differential delay between the 2 inputs.
> I tried it both in trigger mode and in level mode, where the trigger
> threshold is supposed to be at zero crossing (according to the
> manual). The difference between the two is about 0.1nS, which may be
> due to my inaccurate setting of the trigger controls.
One way to see the effect of threshold differences between the 2
channels is to measure the delay between the channels as the signal
transistion (rise, fall) times are increased.

>> However you need to ensure that the reflection coefficient of each 5315
>> input is small.
> How do I do this?
Either estimate it (use a circuit simulator) from the input impedance (R
and C) and the signal transition times or find a TDR (expensive) to
measure it.
You need to know the signal seen by each trigger circuit and also have
some idea of the trigger circuit bandwidth to estimate the significance
of any such reflections.
A circuit simulator can give you some idea of the significance.
Terminating each input in 50 ohms helps but if their is a significant
wiring length between the input connector and the trigger circuit the
reflection coefficient will not be as low as that when the termination
is at the trigger circuit input.
> Thanks very much,
> Joe Gray

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