[time-nuts] Trimble GPS board

Angus not.again at btinternet.com
Sun Aug 23 08:29:45 EDT 2015

Hi Bob,

I got a Trimble 57963-80  last year and the 73090 OXCO and some other
parts on it are supplied with 5V from the LT1764A.

The DSP, FPGA, etc., also have various small regulators supplying
their supply voltages - one even has a dropper resistor in series.
These are connected to the main input supply, so raising it above what
it should be is probably not a good idea.

I thought at first that the LT1764A would be thermally connected to
the fixing hole beside it so that the heat could be removed, but it is
only connected to the copper on the top of board at that point - then
again, that may be the way it was mounted.

I did put a temp sensor on the 1764 to see how much it heated up
during warm up and when running, but neither looked close to being a
problem when run at 6V and room temperature. 
At 6V, it took 2A during warm up and just over 1A when running at room
temp, but the warm up is fast.

When I got the board, someone had written on the OCXO with a marker
pen what the pins were, and the power was marked as 12V, and an EFC
voltage was also written on (about 0.2V from what I measured) which
was all a little weird.

It was listed as having been tested, but since the seller said that
they had no connection info, who tested it and how is anyone's


On Sat, 22 Aug 2015 14:02:35 -0400, you wrote:

>Hi Arthur,
>Thank you for this information.  I have not received my board yet, I 
>probably still have a few weeks to go.  The LT1764 will take up to 20V, 
>but I would never go to the edge.  You could easily do 12V, the only 
>downside is the dissipation in the LT17654.  Use a variable power supply 
>and raise the voltage slowly.  Monitor the temperature with a 
>thermocouple,  IR spot meter (radio shack had one for $10), or just use 
>your finger.  If you can keep your finger on it comfortable for a long 
>period of time, the temp should be OK.  The junction temp is rated for 

>Now on the flips side, my only concern is the regulator is not supplying 
>the oven power.  Most Trimble OCXOs I see on Ebay are powered by +12.  
>When supplying only 6.5V the oven might not get up to temp producing 
>frequency instability and some erratic EFC stats.
>See below, here is a 73090 OCXO (same as on some of those GPSDO boards) 
>powered by +12V.  It's possible the regulator is meant to be powered 
>from +12V, supplying power to the digital logic and to the oscillator 
>portion of the OCXO. Then +12V directly supplies the oven.
>I will post my success and let everyone know how I make out.  I 
>purchased this to have a backup for my Z3801 & Z3805.  Both are on the 
>fritz, I will be post those trials and tribulations on a new thread.  
>Thanks so much for your response.
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