[time-nuts] comparing two clocks

[Jim Lux]

On 2/22/14 5:17 AM, Jimmy Burrell wrote:
> I need some help with a 'noob' question regarding some practical
> examples in some of the NIST literature. When attempting to compare
> two clocks, I'm a bit confused on the subject of exactly how to use
> my counter to compare a delayed clock relative to another. Or perhaps
> I should just say 'comparing two clocks'. Let's take some concrete
> examples.
>
> Let's say I want to characterize my Morion MV89 ocxo using my
> HP5335a. Obviously, I can tune the MV89's 10MHz by +/- 1Hz and feed
> it to the counter's input 'A'. Obviously, I can feed in a second,
> external reference clock at 10MHz into input 'B'.  Suppose, however,
> I didn't have an external reference clock. Can I compare against the
> counter's internal time base by hooking a line from the rear jack
> time base output to channel 'B' input? Or am I making it too
> complicated? Do I simply plug into input 'A' and go?


Just plug it into A and go..  Fire up TIMELAB and see the curves 
revealed..  Just remember that what you're seeing is
  sqrt( ADEV(unknown)^2 + ADEV(counter osc)^2)

If your counter is worse than your Unit Under Test, then you're really 
measuring the counter's performance (which is actually quite fun..if you 
have something you know is high quality)

>
> In a somewhat related question, in this article
> (http://www.wriley.com/Examples%20of%201%20PPS%20Clock%20Measuring%20Systems.pdf)
> where two clocks, both divided to 1PPS, were compared, W.Riley makes
> the following statement, "The two 1 PPS outputs were connected to a
> Racal Dana 1992 time internal counter having 1 nanosecond resolution,
> and the start and stop signals were separated sufficiently in time
> for the counter to function properly".  I wonder what exactly is
> meant by "separated sufficiently in time for the counter to function
> properly" and how one would go about doing this? For example, is
> inverting one of the signals sufficient separation? If not, how is
> this typically done? Delay line?

Inverting would work..

Generally, in this sort of thing, you want to make sure that A always 
occurs before B, so you're counting the right thing, and that you're 
getting small changes in, say, 0.25 seconds, as opposed to getting 0.01 
seconds on one measurement, and then 0.99 seconds on the next.

A lot of 1pps systems (e.g. those not synchronized to an outside source) 
have a "starting phase" that is arbitrary.  You have a 10 MHz crystal 
running to a divide by 1E7.  Turn it on, and you start getting 1pps 
pulses out.