[time-nuts] Trimble GPS board

Bob Benward rbenward at verizon.net
Sun Aug 23 23:36:27 EDT 2015

Hi Angus,
Thanks for your reply.  My original suspicion is the +6.5V rail.  That needs
to come from somewhere, and you need headroom if it's an unregulated input.
Pumping 2A through a regulator like that is no easy feat.  Although rated
for 3A, you need to keep the junction temperature below 125C. The part has a
30C/W thermal resistance (Tja) with a 1" square copper pad.  So 6.5V and 3A
calculate as follows: (6.5V-5V)*3A=4.5W.   4.5W*30C/W=135C RISE, add that to
ambient, 25C, and you are 35C over max junction.  In my world we have a 70C
ambient, and that leaves me (125C-70C)/30C/W=1.83W, or a maximum of
1.83W/1.5V=1.22A.  Generally a tab mounted TO-220 (a D2Pak) can have a Tjc
(junction to case) or less than 1C/W.  It's all the reest of the mounting
that piles on the thermal resistance.

The Trimble board has a top layer pad, some far side pad, and probably
multiple layer of ground plane over the rest of the board.  The mounting
post is mostly likely part of the thermal resistance calculation.  There is
also a time constant involved, it probably can't take the 2A forever, just
long enough to get the oven up to temp. I suspect they may have gotten the
thermal impedance down as low as 10C/W.  I will test it with 12V and let
everyone know if I fry the board.


>>> -----Original Message-----
>>> From: time-nuts [mailto:time-nuts-bounces at febo.com] On Behalf Of Angus
>>> Sent: Sunday, August 23, 2015 8:30 AM
>>> To: Discussion of precise time and frequency measurement
>>> Subject: Re: [time-nuts] Trimble GPS board
>>> 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
>>> voltages - one even has a dropper resistor in series.
>>> These are connected to the main input supply, so raising it above what
>>> 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,
>>> 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
>>> 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 guess...
>>> Angus.
>>> 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
>>> >125C.
>>> >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.
>>> >
>>> >http://www.ebay.com/itm/10-000000MHz-Trimble-Double-Oven-OCXO-
>>> Trimble-7
>>> >3090-Double-sinewave-15V-12V-/251883335405?hash=item3aa56aaeed
>>> >
>>> >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.
>>> >
>>> >Bob
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