[time-nuts] comparing two clocks

[Bob Camp]

Hi

On some counters, if both inputs arrive at exactly the same time, they get very confused. The normal approach is to offset one by a few hundred ns or so. The exact offset is fairly non-critical. It’s real value depends entirely on the amount of drift you expect to see over the time period you are checking. 

If your oscillators are off by 1 ppm, they will slip by 1 us per second. If you want to check them for 12 days or more you will need an offset of more than one second. If they are off by 1 ppb, then your offset could be a bit over one millisecond to handle a 12 day run.  (12 days is roughly 1 million seconds). 

Bob

On Feb 22, 2014, at 8:17 AM, Jimmy Burrell <jimmydburr at gmail.com> 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?
> 
> 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?
> 
> Thank you,
> 
> Jim...
> N5SPE
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