Hi Ed,
First of all, don't be fooled to think it is "calibrated", not yet.
What I've done so far is just starting to make my ways into the GPIB
commands to make it do the right things.
I've not even attempted to do the right thing in terms of actual
calibration, but trying to make sure that I can get it to do the
necessary stuff.
Anyway, one trick being used is to use a locked offset oscillator, that
effectively sweeps all phase relationships over the 10 MHz cycle stable,
predictably and evenly. This way it sweeps over all the phase
relationships to the locked 100 MHz oscillator and the 10 ns time error
window that is then expanded 400 times in the interpolator before being
counted in the 100 MHz clock. The scaling of this is done with an
internal reference pulse. The internal reference pulse needs to be
calibrated with external software, and once established it is put into
the PUD command and used in the internal self-calibration routines.
So, 4.05 ns or 4.29 ns is really a property of that hardware.
I have not attempted to do the routine to establish the calibration
pulse length yet, I'm trying to clear the way before I get there.
Hence, I have not yet cared about actual precision yet.
Cheers,
Magnus
On 02/01/2017 08:46 PM, Ed Palmer wrote:
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
calibration?
Ed
On 2017-02-01 11:00 AM, Magnus Danielson <[email protected]>
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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