Hi Ed,
I am the one who discovered the 1PPS pulse while troubleshooting a
NTG550AA. Instead of reuse the 1/2 PPS output and missing this signal,
my plan is to recycle the 9.8304 MHz output circuitry and connector, the
circuits are almost identical. So I will cut the trace that goes from
TP14 to U405 pin 6 and also use a wire wrapping wire to joint TP14 to
TP33 so the 1PPS will be at J5. I think that I will do the modification
this weekend.
I don't imagine any future use of the X8 Chip signal but having the even
second output could be useful, at least to see the difference with the 1
PPS.
I had not measured the time difference yet, but I made a partial
schematic of the board for my troubleshooting and there I see that the
1/2 PPS signal is synchronized with the 19.6608 signal that is the
source for the 8X Chip ( 9.8304 MHz), this is done in U405B . The period
of this signal is about 50 ns and this is the origin of the 1/2 PPS
width. The 19.6608 MHz oscillator is phase locked somewhere to the 10
MHz oscillator thus it is as stable as this one.
I think that using the other half of U405, which actually is used to
divide by 2 the 19.6608 MHz signal, could render the 1 PPS synchronized
with the 1/2 PPS and also with the same width. Probably the easier way
to correct this is to use the command which sets the antenna cable delay
and compensate for the difference.
I don't have a full schematic, even I am not sure that the partial one
is 100% correct but I can send it to anyone who wants it.
Regards,
Ignacio
El 10/06/2015 a las 6:30, Ed Armstrong wrote:
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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