[time-nuts] High Stability OCXO for Pendulum CNT-90

GandalfG8 at aol.com GandalfG8 at aol.com
Tue Oct 27 04:51:03 EDT 2015

Hi Stan,
The CNT90 service manual is available online from Fluke for their PM6690  
rebadged version and is identical to the original, a search on "PM6690  
Service" will find it.
The manual contains a brief description of the oscillator  and a schematic 
showing both the standard and ovened oscillators, and as  far as I can tell 
from my CNT90 all required components for the ovened  oscillator circuit, 
other than the oscillator itself, are already fitted to the  PCB so just 
adding an appropriate oscillator would probably be all that's  required.
The standard and ovened oscillator circuits are completely separate,  with 
individual outputs feeding the counter circuit and with individual  ON/OFF 
control lines fed back, which suggests that both oscillators might  be able 
to co-exist with auto selection depending on what's fitted.
However, the circuit description states that only one or other of the  
oscillators is fitted, and the same internal calibration procedure is used for  
both, so it may be necessary to disable the standard oscillator, probably 
just  removing the crystal would suffice, but I've not checked this.
Playing with this is another of those things on my eternal to do list  but 
as I've always used an external reference it's never been much of a  
The manual indicates two versions of the ovened oscillator in the  parts 
list, without identifying either, but it should be  quite straightforward to 
wire up sample oscillators for off board  tests to confirm possible 
co-existence and appropriate EFC characteristics  etc.
Plan Z of course might be for anyone who's got a CNT90/91 with the ovened  
oscillator fitted to lift the lid and take a look:-)

I have a Pendulum CNT-90 with the standard oscillator option. I notice that
 there is an open area on the main PCB where a higher stability reference
 would be installed.
 1)      Is it possible to install an aftermarket high stability reference
 oscillator into this stock CNT-90 and be able to use it?
 2)      If so, is it just a matter of soldering an appropriate OCXO module
 into the designated area on the PCB?
 3)      Will the CNT-90 automagically recognize it, or are there jumpers or
 other things that have to be done to enable it? 
 4)      Any recommendations for a good, high stability reference to use?

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