[time-nuts] Trimble Thunderbolt GPSDO Troubleshooting

Charles Steinmetz csteinmetz at yandex.com
Tue Mar 29 04:48:36 EDT 2016


Ryan wrote:

>Also confusing is the quad op-amp seems to be 
>saturated at the negative rail. I can see this 
>Opamp feeds the adjust pin on the OCXO, but I’m 
>not sure what feeds it. Guessing the FPGA? I 
>still can’t find the DAC
 all I see are these 
>pictures [ ] but the pictures aren’t of the DAC, 
>unless the DAC is a resistor ladder (hadn’t 
>thought of that until now). Now I’m starting to 
>realize this is probably the case.

Stewart Cobb described the operation of the DAC 
(actually, PWM) in a post on Nov 2, 2013 
("Subject: [time-nuts] Thunderbolt tuning DAC 
theory of operation").  Check the list archive.

>Does it seem likely the 5V reference is dead?

If the output is at 0.6v, as you say, then either 
the reference chip is bad or something on the 
load side of the 5v reference bus is drawing too 
much current and dragging it down.

>I don’t really want to just lift the output of 
>the 5V reference and leave whatever it feeds floating.

Why not lift the output pin and connect the PC 
trace to the 5v logic supply through, say, 470 
ohms?  This will limit the current through the 
load to ~10mA if the load side is bad.  Then 
measure the voltages on the referrence output pin 
and the PC trace.  One or the other should be ~5v 
(unless the reference and the load side are both 
bad).  The one that is low will tell you which 
side (reference or load) is the problem.

Best regards,

Charles




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