[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
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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