[time-nuts] 10MHz Rubidium reference source for frequency counter

[John Miles]

> Assuming that the test above (with the external REF) gives similar
> results to what you posted today, there is something wrong with your
> measurement setup.  Are the input signals triggering the counter very
> stably near the middle of their peak-to-peak voltages?  Is the
> counter adjusted to give maximum resolution for the sampling you are
> doing?  Does your sample interval give the counter time to process
> each new sample?  Have you read Chapters 3, 4, and 5 of the User's
> Manual carefully, and set up the counter properly for your
> measurements?  Are you sure your TimeLab setup is correct?  (I'll
> leave it to John to comment on that.)

It seems to me that it's just a case of expecting too much from a counter.  This is what a DMTD is for (or any of several other types of instruments ranging from easy homebrew projects to pricy commercial products.)  

To get the best performance from a counter, you need to set up the trigger thresholds well away from ground as Charles says.  Also consider taking TI readings rather than frequency readings.  Frequency readings are OK for initial setup and basic ADEV comparisons but they won't generally yield the best ADEV fidelity or the lowest possible measurement floor.  I have no experience with the Pendulum counters but I wouldn't expect them to do much better than 1E-10 at t=1s regardless of how you set up the measurement, just based on their single-shot resolution spec in the 50-100 ps region.  

By itself, a counter is a good tool for monitoring long-term performance (meaning minutes to days), but it's just not the right tool for measuring the short-term stability of a quality HF OCXO.   For your (Karen's) task, I would suggest taking a look at the tight PLL methodology -- see http://www.ke5fx.com/tpll.htm and other references linked from that page.   I think that's probably the easiest way to measure devices in the 1E-12/s range.  It's an underrated, underutilized technique.

-- john, KE5FX
Miles Design LLC