Hi Ed,
I have started another thread under the name "NTG550AA 1 PPS mod" for
finding the subject easier and I include here my thoughts about this
modification.
I am the one who discovered the 1PPS pulse while troubleshooting a
NTG550AA. For me 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. Instead of removing 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 to
joint TP14 to TP33 so the 1PPS will be at J5. I think that I will do
the modification this weekend and will post the results and pictures.
I have 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 but probably this is
overkill and an easier way to adjust this is to use the command which
sets the antenna cable delay and compensate for the difference.
Checking the specs documentation of a very close cousin of this board,
the GSBW50AA, I found the requirement for the even second pulse: "+/-1
μs traceable to and synchronous with GPS Time Even_Second with at least
one satellite in view". In fact this is something not easy to measure
unless you have a calibrated 1 PPS source.
Another spec states " The falling edge of Even_Second shall occur 0-5 ns
after the falling edge of SYS_CLK." (the 9.8304 MHz signal that is used
as the clock reference in the CDMA system).
This specs are the reason why the even second pulse is synchronized to
the SYS_CLK as I said before.
So my opinion is that the difference that you measured is not relevant
because we can not be sure about the even second accuracy and if we need
to be sure of the absolute time we will need to compare the 1 PPS
output against a calibrated source, maybe another GPSDO that has been
compared with a known standard and compensated for the antenna cable
delay. Of course for a NTP server you have more than enough.
As I said in the other post I have a partial schematic of the board, ask
me if you want a copy.
Best 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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