[time-nuts] Lost Calibration on CNT-81/PM-6681

Ed Palmer ed_palmer at sasktel.net
Wed Feb 1 14:46:14 EST 2017

Hi Magnus,

When you did your measurements, did you use 'single' mode or 'normal' mode?

When I got my PM6681, I wanted to check the interpolater to make sure 
that it was healthy.  I couldn't generate pulses over the whole range, 
but over the range of 50ns to 28 ns, my StdDev readings in 'single' mode 
were in the range of 2.6 - 3.6 e-11, i.e. similar to the example in the 
manual.  In normal mode, my readings were significantly better.  So I'm 
assuming that 'single' mode was the correct mode to use.

FYI, my unit was factory calibrated with a 4.05 ns pulse according to 
the PUD command, so I guess that's what gave the best results.  I also 
see that the example in the manual was for a 4.29 ns pulse. Does that 
suggest that shorter, harder to generate, pulses are important for this 


On 2017-02-01 11:00 AM, Magnus Danielson <magnus at rubidium.dyndns.org> wrote:
> Fellow time-nuts,
> With the hints from the former Pendulum service guy, I have started to
> write my own code in order to restore calibration on a CNT-81/PM-6681.
> This have been a discussion on and off for a couple of years, so I ended
> up buying a PM-6681 which had the Loss of Calibration message "CAL LOSS".
> First things is to replace the CMOS battery, which is a trivial thing to
> replace the CR2032 battery, had one laying around.
> Then, I've been digging into the NI VISA files, which have snippets of
> actual code in it. As I don't have NI VISA and not running with my
> Prologix, I was a bit out of luck there. So I had to start from the
> ground up, taking a serial interface hack I already have, write some
> minimalistic Prologix support for it (TvB hp59309.c provided some needed
> clues on how to get it working stable).
> Then more and more bits and pieces have been falling together, like
> being able to build and write the *PUD string. Also, triggering the
> calibration itself and using an external source.
> Now my counter does not display the error anymore and seems to behave
> more coherently. I'm not completely trusting it, as I am not doing the
> full sweep over calibration pulse calibration values and measuring their
> effect, that will be part of the complete solution, but at least I get
> sufficient part of the way.
> Far from bullet-proof, it is however worth celebrating these baby steps
> in the right direction.
> Cheers,
> Magnus

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