[time-nuts] Frequency Counter Choice

Sun Nov 8 22:01:19 UTC 2020

Old story about the PM6681 (18 Nov 2015, thread: "PM6681 and Timelab")
where a sort of calibration procedure is described: the PUD command is
NOT a calibration command.
PM6681 programming manual, page 9-127: PUD Protected User Data...This
is a data area where the user may write ANY data up to 64
characters...
If the user can write any data, how can it be a calibration command or
calibration data area?
Better watch out those 3V coin cells, we will never get the real
calibration commands/procedure. I have tried with the disassembled
firmware, no way. The visible strings of GPIB commands are all
described in the programming manual, so nothing useful.

On Sat, Nov 7, 2020 at 3:31 AM Magnus Danielson <magnus at rubidium.se> wrote:
>
> Hi Rex,
>
> I need to dig in the archive to refresh my memory. I don't recall
> precisely, but I think I recalled that the manual indirectly describes
> the calibration data string.
>
> I have learned a few things from Pendulum, but I did not have the right
> tools at hand to set things up.
>
> There was a more recent setup that could use more modern generators, but
> the trick was still the same. You lock the generator and counter to the
> same frequency, then you set the generator to a small offset frequency
> from 10 MHz, which is 9.999 MHz as I recall it. This slowly sweeps
> through all the phase-relationships between the reference oscillator and
> the counter input, thus sweeps the interpolator phase. It then chooses
> the calibration constant giving the lowest RMS error, as this is the
> best compensation for the hardware min-point. All this is free from
> memory. Then that value with calibration date is written into memory. If
> I recall correctly 2.21 ns is a typical value.
>
> I have PM6681 in need of calibration, and as I recall it I was able to
> program it enough for the calibration error warning did not show up.
> This not to say it was actually calibrated.
>
> At some point I will return to that project. The generator I used did
> not support that offset frequency, but I have others that do. Also, my
> crapiola GPIB programming needs attention. My intention is to share the
> fruits of this project when it comes to that. The lab has been in
> shambles for too long, but shaping up slowly and nicely.
>
> Cheers,
> Magnus
>
> On 2020-11-05 09:30, Rex wrote:
> > Hi Magnus,
> >
> > Just catching up on list messages and saw this one from you.
> >
> > I have a Fluke PM 6881 counter. I don't think I've ever seen a
> > description of a method for reading/restoring these battery backed up
> > calibration constants. I looked for a way, as losing them is something
> > I've worried about. Not that it has happened and I did replace the
> > battery once.
> >
> > Is doing this described in one of the manuals? Sounds like it is
> > through GPIB?  I'd greatly appreciate any pointers to info or other
> > details you might provide.
> >
> > I did see, in the service manual, a short description of a method for
> > Interpolator calibration that seems to be for making these
> > calibrations. Seems if the saved cal values get lost, the counter will
> > display "CaL.LOSt". The cal procedure is driven by an old DOS program
> > (that I've never found) and requires a: PM5768 Pulse gen, PM5193 LF
> > Sig Gen, good 10 MHz, all GPIB controlled from the program. Never saw
> > more details but sounds messy. If there is description of GPIB
> > commands for reading/setting cal values, I missed them.
> >
> > thanks for mentioning this and anything more you can provide
> > -Rex
> >
> > On 10/29/2020 5:37 PM, Magnus Danielson wrote:
> >> Hi,
> >>
> >> I second this. You can read the calibration data out of the counter and
> >> save. I've done some experiments with that, but nothing conclusive, but
> >> I blame my lack of patience and not a proper setup.
> >>
> >> Do replace the battery, it is cheap and relatively easy to do.
> >>
> >> Would you loose this calibration, through a little GPIB commands one can
> >> write a fake value in. This will however not produce the best resu. The
> >> calibration routine actually runs an off beat frequency and then test
> >> different values, and look for least RMS value, because it is the
> >> calibration point. I've not had time to replicate all that, but I did
> >> manage to write the fake value in and at one time get rid of the CAL
> >> LOST warning.
> >>
> >> Cheers,
> >> Magnus
> >>
> >> On 2020-10-29 14:17, Azelio Boriani wrote:
> >>> For those who have the PM6681 (aka CNT81): check the 3V memory backup
> >>> cell and replace it before the dreaded calibration lost (CAL LOST)
> >>> will appear on the LCD. Replace the coin cell with great care (with
> >>> the counter powered up), see the service manual for the procedure.
> >>> <https://archive.org/details/FLUKE_PM6681_Service_Manual>
> >>>
> >>> On Fri, Oct 23, 2020 at 6:17 PM Magnus Danielson
> >>> <magnus at rubidium.se> wrote:
> >>>> Hi,
> >>>>
> >>>> On 2020-10-22 19:13, Attila Kinali wrote:
> >>>>> On Thu, 22 Oct 2020 11:50:08 +0000
> >>>>> Giorgio Barinetti <giorgio at barinetti.it> wrote:
> >>>>>
> >>>>>> Choices are many, but I'll try to avoid the "older" machines lile
> >>>>>> 5370 or 5335. The 531xx series seems nice ( money apart )
> >>>>>> But again : which one between the 3 ? 53131, 53132 or 53181 ?
> >>>>> Maybe try to get hold of one of the Philips (later licensed to
> >>>>> Fluke) PM6680 or PM6681? These are more common in Europe than
> >>>>> in the US, so the big US dominated websites/forums/.. don't
> >>>>> mention them that often. Solid devices that can be had as low
> >>>>> as 300€ if you are willing to wait, 500-800€ is the usual going
> >>>>> price. The SR620 is the workhorse that drives a lot of the
> >>>>> time and frequency metrology worldwide and can be had new and
> >>>>> used (new on http://thinksrs.com goes for 800-2000€ used).
> >>>>>
> >>>>> If you go for a new one, I would consider looking at the
> >>>>> Pendulum CNT-90 and CNT-91. (Pendulum is the company that
> >>>>> took over Philips frequency counter business and the CNT-90
> >>>>> is the continuation of the PM668x line, also sold as PM6690
> >>>>> by Fluke)
> >>>> Let me correct on the history and geniology there.
> >>>>
> >>>> Philips had a instrument making side called Philips Industrier
> >>>> Järfälla
> >>>> that did a range of measurement instruments. Later they joined forces
> >>>> with Fluke. Later Philips felt that the business unit was a bad fit to
> >>>> stay in Philips, so they sold it off to become a separate company
> >>>> which
> >>>> became Pendelum. Pendelum was really the business unit with people etc
> >>>> through that process, and the Fluke relation and rebranding continued.
> >>>> Naturally Pendelum moved out of the Philips Industrier Järfälla office
> >>>> over to Bälstabro (both locations in north of Stockholm) as it was
> >>>> sold
> >>>> off. Pendelum also managed to rebrand their counters to Tektronix,
> >>>> which
> >>>> mainly consisted of cosmetic changes to get the look and feel.
> >>>> Pendelum
> >>>> was operated for many years like this, some of their production in
> >>>> Pajala, where as other where done in Bälstabro. Later they reshaped
> >>>> the
> >>>> production so that it moved to Poland where it remains. Pendulum was
> >>>> sold to Spectracom and was operated as a subsidary for a while, until
> >>>> they shut operation down.
> >>>>
> >>>> The CNT-80/81 (PM 6680 and PM6681) production went on as long as they
> >>>> had the timing ASIC. The CNT-90 (100 ps) was developed to the
> >>>> CNT-91 (50
> >>>> ps), where the later replaced the CNT-81 (50 ps). They aimed to do the
> >>>> CNT-92, but could not at that time do it with the same technical
> >>>> setup.
> >>>> They also had the Wander Meter WM-10 which aided in testing telecom
> >>>> sync. After some testing, I suggested they would broaden the
> >>>> product to
> >>>> handle more signals and that is when they mostly firmware upgraded
> >>>> it to
> >>>> the WM-11, and first time I tested the WM-11 the second '1' was cut
> >>>> out
> >>>> from another '10' print and put over the '0'. Also, it was still under
> >>>> development. Several years later they where still going back to my
> >>>> list
> >>>> of feedback. They later did a revamped this into a new instrument with
> >>>> even improved capabilities such as interfaces boards. This was later
> >>>> sold off to Calnex which sells it as Sentinel, which is a great
> >>>> product
> >>>> for telecom operators.
> >>>>
> >>>> I still have contact with some of the Philips/Fluke/Pendelum staff and
> >>>> owner.
> >>>>
> >>>> Cheers,
> >>>> Magnus
> >>>>
> >>>>
> >>>>
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