[time-nuts] Trimble Thunderbolt GPSDO Troubleshooting
Charles Steinmetz
csteinmetz at yandex.com
Tue Mar 29 08:48:36 UTC 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 Im
>not sure what feeds it. Guessing the FPGA? I
>still cant find the DAC
all I see are these
>pictures [ ] but the pictures arent of the DAC,
>unless the DAC is a resistor ladder (hadnt
>thought of that until now). Now Im 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 dont 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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