Hello Time-Nuts,

A while back I acquired an HP 53131A, and a bit later added one of the
Chinese knock-off 3GHz prescalers.  Makes for a nice counter except
for the very poor standard timebase.  So, While researching the Option
010 and finding how outrageous the prices are running I found an
Option 010 DIY project.  It occurred to me that I could just
substitute a Symmetricom X72 rubidium oscillator for the ovenized
OCXO.

It came out really nice (and is much better than an Option 010 - at
least for anything over 10-100 seconds).  I have a header so that I
can calibrate the oscillator over the serial port (a one-time exercise
I imagine).  And I added a LOCK indicator in one of the removable
plugs right above the reference BNCs on the back panel.  Both the
oscillator and the LOCK light are powered from the always on +12V so
there is no warmup when the counter is turned on and the LOCK
indication works when the counter is powered off.  And since the fan
runs 24/7 I'm not too worried about the X72 overheating.

All the extra hardware is mounted on a simple 'L' shaped aluminum
plate that bolts where the Option 010 mounts to the chassis side rail.
I used a 16-pin flat cable that plugs directly into the 53131A main
board.  The only modification that was made to the stock counter was
to drill the hole in the plastic plug for the LOCK light.

Now the Reference Out can also be used to drive other devices as well
(with reasonable accuracy).

Relatively easy project that can be assembled in a weekend .

Pictures are attached.

Skip Withrow
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