[time-nuts] Frequency Counter Choice

Mon Nov 9 09:58:33 UTC 2020

Thanks for the memory refresh.

You can read the string using PUD?

Do that and keep the result. PUD and PUD? is the magic in the counter,
the rest is software and hardware outside of the counter for calibration.

In order to write, you need to move the calibration jumper inside.

Cheers,
Magnus

On 2020-11-08 23:01, Azelio Boriani wrote:
> 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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