# [time-nuts] comparing two clocks

Volker Esper ailer2 at t-online.de
Sat Feb 22 12:50:08 EST 2014

```Jim,

If I get you right, you want to compare the 10MHz outputs (not the
1PPS). As Jim and Bob told us so far, the thing is to provide, that
input A _always_ starts before input B (or the other way around).

Connect the signals to an oscilloscope, and check, how much the phase
differs - if the rising slopes occur close together, put some
meters/yards of coaxial cable into one of the two signal paths. 1 meter
is roughly worth 5ns - while the period of 10MHz is 100ns, 1m cable will
phase shift about 18 degrees. I didn't verify, if the coax cable (with
it's microphonic effect) affects the ADEV - does anybody have experience
with this? Otherwise I'd have to fire up my counter and have a
measurement on the run...

Of course, inverting one signal will do as well. If you do it with extra
electronics that definitely will affect the ADEV. I find it much easier
to use some meters of cable.

Ok, my counter is heating up by now...

Volker

Am 22.02.2014 14:17, schrieb Jimmy Burrell:
> 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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