[time-nuts] Basic TIC measurement

[Magnus Danielson]

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.
> 
> 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 HP 5315A is using the bare MRC chip without external interpolators, 
so that's why you only get the 100 ns singel-shot resolution, i.e. the 
bare 10 MHz coarse counters. The MRC chip is used in 5314, 5315, 5334 
and 5335 counters to name a few. I gave away my 5315 counter to a friend 
who had no counter, but I still have a 5314 counter (and 5334 and 5335) 
around.

With averaging and the 100 kHz and 10 MHz not being locked to each 
other, the asynchronous clock beating will cause the averaging to have a 
meaningfull effect.

> 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, rather the opposite. There is an overbeleif in using PPS signals. 
PPS is good for one of the signals, but 100 kHz is good for the other.

> 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.

No loading means the current can charge the capacitance quicker.

Cheers,
Magnus