# [time-nuts] comparing two clocks

Jimmy D. Burrell jimmydburr at gmail.com
Sat Feb 22 19:10:38 EST 2014

```Hmmm... Magnus thank you for your comments. The 5 ns figure is certainly helpful. My 1980 manual from HP just says "Time Interval" A -> B = 0 ns to 10E7 seconds. Not very helpful.

Many thanks,

Jim...
N5SPE

On Feb 22, 2014, at 5:20 PM, Magnus Danielson <magnus at rubidium.dyndns.org> wrote:

> Hi Jim,
>
> On 22/02/14 14:17, 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?
>>
>> 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?
>
> The problem is that the start trigger (Channel A) will arm the measurement on the stop channel (Channel B). For this process to operate properly, allow 5 ns. If you look into the 1992 spec-sheet it says that TI interval is from 5 ns.
>
> If you have a 10 MHz or 5 MHz signal, you only have 100 ns or 200 ns periods to play with. Dividing them down to say 10 Hz gives you plenty of time such that you can boot-strap one of the dividers such that you don't go through zero within the measurement period.
>
> Adding a delay doesn't help, as frequency error will get the phase into through zero condition pretty quick I would guess.
>
> The SR620 counter has a coax delay-line to give trigger look-ahead.
>
> Cheers,
> Magnus
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