[time-nuts] Basic TIC measurement

[Bruce Griffiths]

Joseph Gray wrote:
> Another newbie question (sorry). I'm using an HP 5315A Universal
> Counter and doing some basic experiments to teach myself.
>
> Here is the setup: 100KHz square wave as the "A" input to the 5315A.
> Using a BNC-tee to connect a length of RG58A/U from "A" to the "B"
> input and another tee with a 50 Ohm terminator. Using the TI Avg mode
> and the gate time maxed at 13.4S, I read a TI of 22.5nS.
>
>   
What about the difference in propagation delay between the 2 counter inputs?
This could easily amount to several ns and account for your results.

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.

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.

> Assuming I got the numbers correct, light takes 3.3356nS to travel 1M
> in a vacuum. 22.5nS / 3.3356nS/M = 6.745M. If I divide the measured
> length of the coax (3.825M) by 6.745M, I get a velocity factor of
> 56.7%. This is quite a bit lower than the nominal 66% I expected.
>
> So, is the difference due to:
> The BNC-tee connectors?
> My use of a 100KHz square wave instead of a PPS?
>   
It shouldn't matter as within the resolution of your instrument the
cable is nondispersive.
However you need to ensure that the reflection coefficient of each 5315
input is small.
> The resolution of the 5315A (100nS single-shot, 1nS averaging)?
> Some other failing of the 5315A? It was just out of cal in Feb 2009.
> Something else entirely?
> All of the above?
>
>   
Just the differential delay between the 5315 inputs.
> The reason I'm using a 100KHz square wave is that I haven't yet modded
> either of the Z3801A's to bring out a PPS signal where I could use it.
> That is on my short list. The 100KHz from a Spectracom 8140 was the
> closest I can get right now. Is this type of measurement useless with
> anything other than a PPS signal?
>
>   
No, as long as cable dispersion is insignificant.
> An interesting observation - when I remove the 50 Ohm terminator, I
> get a much shorter TI. I assume this is due to reflections back down
> the coax.
>
>   
Yes this will alter the signals seen by each input and due to the finite
slope of the input signal and differences in trigger threshold between
the 2 inputs the effective differential delay will change.
> Joe Gray
> KA5ZEC
>
>   
Bruce