Hi, this is my first post ever to a mailing list, so if I'm doing
anything wrong please be gentle with your corrections :-)
A short time ago I purchased a Nortel/Trimble NTGS50AA GPSTM, I'm sure
many on this list are familiar with it. At the time of purchase, my only
interest was the 10 MHz output, for use with my HP5328b frequency
counter and perhaps in the future also my signal generator. No question
here, it just works great as is. However, it certainly seems best to
leave these devices powered up all the time.
OK, now were getting close to my question. The unit pulls about 10-11
watts, which is really not very much. But it kinda bugs me to have it
sit there using electric and basically doing nothing when I'm not using
it. So, I bought a Raspberry Pi 2 with the intent of using it as an NTP
server. I can't really say I'm enjoying my intro to Linux a whole lot,
but I'll get there. It still needs some work, but it does function with
the PPS output from an Adafruit ultimate GPS, which I bought for testing
this and possibly building my own GPSDO in the future.
The NTGS50AA is a very capable device, but unfortunately it does not
have a PPS output. Instead it has an even second output, which goes low
for approximately 50 ns. The falling edge of this pulse marks the
beginning of the second. During my search for a solution to this, I came
across a post from this mailing list which I believe was discussing
repair of one of these units. Someone in that post mentioned that there
was a PPS signal at test point 33 which went low for about 10 µs. Thank
you, that saves me a lot of probing.
The first thing I did was verify that this pulse did exist, then I
decided to examine it a little closer. I kind of suspected that it may
have been a rather raw pulse as received from the satellites. I found
out that is not correct, once the unit successfully phase locks, this
PPS signal is very accurately tied to the 10 MHz output, even when the
unit goes into holdover mode. I was very happy about this :-) Next step
was to see how accurately it was synced to the even second pulse. The
bad news is that it does not occur at exactly the same time as the even
second. The good news is that the offset is very consistent, 253 ns
before the even second pulse, +/- 1 ns.
My next step was to find out where the even second pulse entered the
output circuitry. I then broke the trace taking the even second into the
output circuitry, and ran a piece of 30gauge wire wrapping wire from the
via at test point 33 to the via at the input to the output circuitry.
The wire fit so perfectly it felt like the vias were made for just this
purpose :-) Now I've got a very nice PPS signal available both at the
front jack and at the backplane connector in the rear of the unit.
OK, here is the actual question. Do you think it is OK to consider a
pulse which arise 250 ns early to be close enough? And no, I am not
forgetting about that 3 ns, there is about 3 ns of delay added by the
output circuitry.
Hope you didn't mind the long-winded post, and I thank you in advance
for any advice you offer.
Ed
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