Fluke PM6690 12 digit/s frequency counter

[DaJMasta]

I snagged a used (and somewhat beat up) one of these counters and looked around a bit - doesn't seem to be a lot of info outside of the official docs (which do include a schematic!)  I figured I wanted to clean it up and check things out for myself anyways, so why not post pics.  I've also heard it suggested that the PM6690 is a rebadged Spectracom CNT-90, and given the look of the front panel and the specs, it did seem likely.  Well I can say for certain that is the case 

It's evident that my counter has been punched, scraped, and stuck to quite a bit in its lifetime, but best I can tell it's still in great shape.

And it's one of those long form factor units, next to it is a Siglent SDG2082X function gen


The unit powers up when plugged in and the fan starts whirring.... sort of annoying.  Over the next few minutes it cranks up at least 3 times to be rather loud for such an instrument.... but at least it's off in standby (though I'm sure the OCXO is still going)

The model number is a PM6690/AN, which isn't listed as an official spec in Fluke's datasheet... it's supposed to have a three digit code not a two letter one.  Also fluke never mentions a 2.7GHz channel C option, they start at 3GHz.  So I navigate to the information menu...


...and the installed option codes aren't listed on Fluke's datasheets..... but they are listed as valid options for the CNT-90.  The option 30 timebase is their mid tier OCXO (a notch under the best and two up from the base) and option 10 is the 3GHz input C - not the 2.7GHz specified on the front, but in the datasheet it breaks down the input voltage range for different bands and 2.7-3GHz is the lowest - maybe that wasn't to Fluke's desired spec so they said only 2.7GHz?  Also the firmware is from 2006, as is the boards, I found after I opened it up.

Of course, I knew the screen had some damage, but I didn't expect the backlight to die in the first hour - was making odd waving patterns in the beginning, then went to half brightness for a bit, then just went out.


It was time to dig into the thing.


Pretty clean inside, pretty solid construction, no loose bits and no obvious damage or mods (the cal sticker had been broken by someone before me!)  Also another confirmation that it basically is a CNT-90:


Thanks Bjorn!

The bottom of the board is just some decoupling:


The input C board:


The timebase, a quick google doesn't give me much, but given where the performance is and the option code, I have a good idea.


And hiding under the PSU there's actually a few chips... including this Sharp ARM SoC running the thing.



So I took off the front board with 2 screws (it's been all torx so far) and took a look.  The schematic made it easy to find the backlight power, and it's just 5V to some LEDs built into the screen.  I follow the trace and.... the solder joint is dry and looks cracked, one more on the other side of the screen too.  So a little flux and solder later, and the backlight is back on.  Still have some wavies in the screen (I scope probed it when it was dead and the power line was stable, but may be worth probing again), but the light has been going for hours and seems fine to me.

The noisy fan is actually a decent fan (NMB) that is pretty quiet uninstalled.  there's no vibration damping and because it's right up against the chassis, the fan grille actually blocks a lot of the airflow and makes most of the noise.  I have a quieter one I want to install instead and will use some rubber washers and a little extra distance as well, but I'm surprised the thing needs a fan.... the highest temps I could find on the board were high 40s Celsius with the noncontact thermometer.  Maybe they just have it to make sure things don't heat up too much for the OCXO when in a bad environment.  And these high 40s, low 40s on the FPGA, were enough to kick the fan up a couple of notches even with the top off.

I've got it measuring my GPSDO output and it seems to be pretty spot on for the specs (I though the GPSDO is not characterized or Cal'd, so it's unofficial), even with the top off the drift doesn't seem to be too much.  I'm quite happy with it so far, will have to hook it up to USB and see if there's some new firmware and whatnot, then see how it triggers off other waves with the function gen - maybe see how much the function gen jitters by piping the GPSDO into both of them.

[DimitriP]

...it also looks like your power supply caps woll need some attention ...sooner or later

[TiN]

I was looking for CNT90 some time ago, but decided to skip. Ain't it 14 digit resolution though?
Nice clean construction and fun to see impedance matching cutouts on ground plane on the bottom

[DaJMasta]

The CNT90 and 91 are both 12 digit/s counters, but both have 14 digits worth when the gate time is long enough or when you're averaging a bunch of samples (the pic with no backlight is 14 digits, but it had been going for a bit).


That front cap does look a little suspect in the pic, but it seems less in person - maybe the camera angle?  Anyways they're Nippon Chemi Cons and it's only 10, I at least hope those caps still have some life left in them!


Looking for the part number on the input C board, apparently this is an option for an earlier fluke (Phillips?) counter that actually did top out at 2.7 GHz.  My guess is they had some left over from the previous generation and wanted to use them, but didn't have so many as to make it an official SKU in the more recent documentation.  Since it probably performs nearly like the Pendulum equivalent 3GHz option board, it probably just identifies as one of those in the firmware.  So this particular unit may actually be slightly different than just a rebrand.

[bitseeker]

Aren't models with /AF or /AN suffixes often an indication that it's for the military? Ah, yes, this counter has an NSN: http://www.parttarget.com/6625-01-530-9444_6625015309444_PM6690-AN.html/-4ED76C21-2112-4800-AF96-895C96EA4505

Perhaps that would account for the variations you see from normal SKU.

Nice job with the fixes and lots of pictures.

[BloodyCactus]

I just picked one of these up, mine has a firmware of 1.21s dec 2007, and timebase 030 from october 2008, same options 10 C input too.

Its a nice step up from my racal dana 1992.

[DaJMasta]

I imagine it is!  I wonder if there's a way to get firmware online from fluke, a basic search hasn't shown anything, but I haven't actually tried contacting them.

Haven't had particular problems with my firmware, so there hasn't been a big reason to look.

I've done a bit of programming since the first post and the VISA drivers seem to work pretty well.  The examples they provide in the programming manual use directional quotes which don't compile properly if just pasted into a code document, but it seems to respond fine to all the commands I've tried so far.  I suppose it's to be expected, but it's nice to know it all works.

[TiN]

I just picked one of these up...

Oh, that was you for 300$ one lately. I wanted it , but forgot to bid....  GAS outbreak prevented... 
Let's see some pics!

[raileon]

I imagine it is!  I wonder if there's a way to get firmware online from fluke, a basic search hasn't shown anything, but I haven't actually tried contacting them.

The Spectracom CNT-9X v1.28 update worked just fine on my PM6690.

https://spectracom.com/get-help/product-software

Check the doc file in the zip. The Fluke PM6690 is listed as a supported product.

[BloodyCactus]

I imagine it is!  I wonder if there's a way to get firmware online from fluke, a basic search hasn't shown anything, but I haven't actually tried contacting them.

The Spectracom CNT-9X v1.28 update worked just fine on my PM6690.

https://spectracom.com/get-help/product-software

Check the doc file in the zip. The Fluke PM6690 is listed as a supported product.

cool!

@TiN
not much difference inside from the above pics

I think, more pins on the prescaler connector


different OCXO. MV197. I believe the upgrade is MV209

[DaJMasta]

Well after a hair-raising 15 minutes, my 6690 is also running the 1.28 firmware.


Went through the normal update procedure, found the instrument and firmware just fine, clicked the button.  Loaded onto the unit, updated the unit.... at about 85% of the progress bar it hung and the screen went from the normal updating screen to the center saying "firmware updating" but the animated icon stopped and the outside of the screen looked like the normal UI....

Gave it like 15 minutes and no progress, the updater had also hung.  Powering down the unit from the front wasn't responsive, removing the USB cable didn't make it responsive... had to pull the plug.  Plugged it back in and it auto-ons, plugged the USB back in.... normal UI loads, and when the USB reconnects, the updater says it finished properly.

Well the about screen confirms the new firmware and it's back to working as I expect.... so i guess it was actually successful 

[BloodyCactus]

does the model retain its identification in the about as a FLUKE PM6690 or is it now a CNT-90?

[DaJMasta]

Still IDs as a PM6690 in the menu and over USB.

[BloodyCactus]

yeah I ran it, device rebooted and it shows updated to 1.28s. I am currently running the Calibrate Internals command it suggested.

[DaJMasta]

Finally got myself an RF generator with a good bit of bandwidth so I decided to check out the performance of my 2.7GHz input C.  At 0dBm it will go up to 3.067GHz, but at +10dBm, I get a cool 3.089GHz.  Not bad for a 2.7GHz input.


Also from browsing around and looking at counters that are available, I think I can match the Tektronix MCA series counters to the CNT-90 (same as this one), and their FCA series is the CNT-91 based version with a 50ps timestamp resolution.  In terms of commercial availability, it seems like this and Keysight's 50ps timestamp are second only to the Keysight 53230a counter - same 12 digits a second, but a 20ps timestamp resolution.  I think I've gone far enough over the timenut threshold to drool over it, but at at least $3000 used, it's not really on my radar 

[coromonadalix]

lucky you   

[volvo_nut_v70]

Having just bought one of these guys I noticed that there is a calibration utility called for in the manual referenced as PM6690.zip. The kind folks at Fluke searched their basement and lo and behold they found it! This should also work on the Pendulum CNT-90, CNT-91, Tektronix FCA3000, FCA3001, FCA3020 and MCA 3000.

Unfortunately it's too big for adding as an attachment, so it's on KO4BB site:

http://www.ko4bb.com/getsimple/index.php?id=manuals&dir=07_Recent_Uploads

As a side note, it had no option C so I added a PM9624 clone off Ebay, seems to work fine! Still have to update the firmware.....

Anyone ever replace the STD timebase with an OCXO, if so which one did you use?

[DaJMasta]

I know there's a PM66XX compatible OXCO upgrade available on ebay, I believe Dave's got a video of putting one in a PM6685 or something similar, and I think the header in the space next to the power supply would be it, though in my counter it was directly populated on the board.  It's supposed to work on PM6690s.

Since that's the case, though, You may be able to just drop in an OXCO with a compatible pinout directly into the board, it looks like the upgrade modules don't have much else on the board besides the oscillator, but it would be work at least looking over the board to make sure yours has the parts around it still populated.

[BloodyCactus]

mine is soldered directly to the board, I worked out what replacement would be. Im not sure if there is a header there or not (I didnt see one when I looked, from memory...). I think a MV209 would work to upgrade to OCXO. I just plug my rubidium source into it.

[Jörg]

Hi, I have tried the fluke PM 6690 Utiliiy with my TEK FCA 3000 .  Basically, the software that works.
After I was asked my serial number and the confirmed to enter, the FCA 3000 rebooted. So far so good, the PM6690 utility has been renamed my TEK FCA 3000 .
On the display and via the USB port, the TEK logs now with fluke PM 6690. I found there is no way to undo that. The programmer manual shows no command to write the IDN. But somehow it should work. The utility has done it without prior notice.

[picitup]

Hi All

I've just bought a Pendulum CNT-90 but I can't seem to get it to work as expected.  I'm inputting a 10MHz square wave @280mv pp from CH1 on my Siglent SDG1025 which is clocked by my BG7TBL gpsdo into the 10Mhz Ext Ref input.  The display shows EXT ref to show it has accepted the signal.

The I connected CH2 of my sig Gen to Input A on the CNT-90 and supplied a 10MHz sine wave at 5v.

The issue is that whatever gate speed/resolution I choose, the last couple of digits on the CNT-90 dance around by about 20 counts.  I had assumed that if the extref is essentially the same clock, the display should be crack on and the display would be. for example, 10.000 000 000 000 but even on faster gate times like 100mS, the last couple if digits dance around, effectively showing more error for faster gate times.

I've played around with different voltage levels for ext ref and sine/square and while these do make a small difference, the display doesn't zero.

Am I missing something here?

Thanks

Steve

[picitup]

Oh, I forgot to say, performing the same test on my TF930 with 100 second gate/10 digits, the display is zeroed as expected, eg:

10.00000000

And is rock solid.

[jpb]

Hi All

I've just bought a Pendulum CNT-90 but I can't seem to get it to work as expected.  I'm inputting a 10MHz square wave @280mv pp from CH1 on my Siglent SDG1025 which is clocked by my BG7TBL gpsdo into the 10Mhz Ext Ref input.  The display shows EXT ref to show it has accepted the signal.

The I connected CH2 of my sig Gen to Input A on the CNT-90 and supplied a 10MHz sine wave at 5v.

The issue is that whatever gate speed/resolution I choose, the last couple of digits on the CNT-90 dance around by about 20 counts.  I had assumed that if the extref is essentially the same clock, the display should be crack on and the display would be. for example, 10.000 000 000 000 but even on faster gate times like 100mS, the last couple if digits dance around, effectively showing more error for faster gate times.

I've played around with different voltage levels for ext ref and sine/square and while these do make a small difference, the display doesn't zero.

Am I missing something here?

Thanks

Steve

I quickly did the experiment on my FCA3100 with a 10 second gate (to get enough digits) and it flips between 9.999999999etc and 10.00000etc:

See the attached picture.

I did set the sine to only 500mV, perhaps 5V is a little high.

[jpb]

When I extend the gate time to 100 secs to get 13 figures then on the first reading it is 9.999 999 999 995.

Your Siglent generator may not be as stable as my Agilent one - if you have a splitter you could use that instead (so it is exactly the same signal).

Also remember the TTi just does whole counts while the CNT-90 is interpolating - the TTi will ignore the noise until it gets as big as a whole count.

But even so, I'd expect your results to be a bit better than you describe.

[DaJMasta]

I think what you're probably seeing is actual noise in the measurement.  Even when clocked from the same reference, that reference is used to control the PLL of an internal oscillator, so if it's averaging that at all or if the reaction time of the PLL isn't instant (and it isn't), there can be some noise that appears if the reference is changing at all (and even the OCXO in the BG7TBL does slightly - you can actually see shifts in the frequency with the DAC steps when measuring with this counter against a more stable reference).  There's also potential for noise in the triggering mechanism, and then there's the fact that you simply can't see the detail you see as fluctuating on the TTI counter - 20 counts on 12 digits is well under the noise you'd be able to see on a 10 digit display, so while there is some inherent noise in the counter (and the data sheet specifies it at length), I think the noise you're seeing is actually a product of the test system.

Just took my PM6690, 1 second gate time, auto trigger, frequency measurement of one of the distribution amplifier outputs of my PRS10 rubidium reference which is disciplined to a GPS PPS signal, which another of the distribution amplifier outputs feeds the external reference, let the counter warm up for a bit, and took some measurements.  In almost 1000 measurements, I'm seeing a peak to peak variation of less than 4 least significant digits at 12 digits per second, and an Allan deviation of 0.6 digits per second.  That's 2 and 3 digits below the displayed least significant digit of the TTI counter, so of course you're not going to see them.

Then, to get your 12 digits on the TTI, it's averaging the counts over the whole time, so the displayed mean in the images I took is actually the same sort of measurement, again showing a smaller noise figure to the point of not being able to be measured by the TTI, 2 uHz out at a 10MHz frequency is 2x10^-13... 0.2 parts per trillion