[time-nuts] Fluke/Pendulum Counters - Rubidium Timebase

Ed Palmer ed_palmer at sasktel.net
Tue Jan 17 11:13:26 EST 2017

Thanks for the info and please pass my thanks on to Stefan.

Yes, I usually do use an external timebase for the PM6681. I need to do 
some troubleshooting on the  unit and thought I'd do a timebase upgrade 
while I was in there.  I want to make some measurements that are 
independent of my house standard (an Efratom FRK).

I haven't quite decided whether the upgrade will be an OCXO or a Rb.  I 
have both LPRO and X72 Rb units sitting on the shelf so the cost will be 
minimal.  If I go with a Rb, I'll just find a small power supply that 
will fit rather than looking for the distribution board.  There appears 
to be enough info in the service manual that figuring out the 
connections shouldn't be a problem.


On 2017-01-17 8:14 AM, Magnus Danielson wrote:
> Ed,
> On 01/16/2017 11:01 PM, Ed Palmer wrote:
>> >I have a Fluke PM6681 counter that just has the basic oscillator.  I was
>> >thinking of upgrading it to the Rubidium timebase.  From the service
>> >manual, it appears to use an LPRO, but there's nothing obvious regarding
>> >heat sinking.  Does anyone have pictures of this installation or, at
>> >least, seen it?  Is there any heat sinking at all?
>> >
>> >I realize the firmware won't report it as a PM6681R and there will be a
>> >few non-standard work-arounds required, as well as an auxiliary power
>> >supply (the original needs this as well), but I don't see any
>> >showstoppers here.  Does anyone have any warnings or advice?
> Asked the former Fluke/Pendulum service engineer Stefan Ledberg, and
> here is his comments:
> The built in Rubidium can use basically any source and have on earlier
> models used a Datum or Efratom LPRO-101 model (no heatsink), and later
> models used Spectratime LPFRS-01 special Heatsink and adapter from DSUB
> to LPRO-equivalent. PSU is added internally that is sharing PCB with the
> output amplifier for the additional 10Mhz output on the rear panel. The
> PSU and output is no longer in production and I doubt there are any left
> at the factory... I can make an inqury if really important, Internally
> 10Mhz is connected to a 2 pin header and Source is selected with a
> jumper. however as stated firmware will still claim std or ocxo
> timebase.  However my recommendation is to power a Rubidium timebase
> externally with off the shelf PSU and just feed the PM6681 on "Ext Ref
> in" this will accept most levels of Signal and no fuss needed.
> Cheers,
> Magnus

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