[time-nuts] Basic TIC measurement

Joseph Gray jgray at zianet.com
Wed Oct 7 02:29:22 UTC 2009

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.

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?
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?

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?

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.

Joe Gray

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