Hi Ed,
While your board is not exactly the same as mine I think that the
schematics are almost identical but the TPs and probably the chip names
has changed. I think that the main difference is that your unit can be
powered with -48 V or +24 V and obviously the parts layout has changed.
I have made a partial schematic and it is a work in progress but it has
been quite useful for me so far. Since it was started when
troubleshooting my board, I focused in certain areas and I still
continue from time to time. The troubleshooting effort, while some
initial success, was not able to get good results and now I have a dead
board that I can trace for expanding the schematic without much care .
I use Eagle to draw the schematic and I'm in the process of adding some
chips to Eagle since they are not included in the standard library,
when I finish I will add a new section which covers the signals that
goes to and from the 110 pin connector J2. I think it will be ready in
a couple of days.
I plan to upload an Eagle .sch file and an image to Dropbox, I will
inform you when it is ready. The Eagle file could be useful for
continuing the reverse engineering effort.
I'm working also in pictures of the board's top and bottom with the
resistors and capacitors labeled according to the schematic, I
registered the top with a mirror image of the bottom and this is a good
way to find the correspondence of the vias but it is advancing at a
slower pace, it takes a lot of time. Also I have made a table with the
signals of all the the test points, I will also include this.
I will keep you informed of the progress.
regarding the delay, I checked that Lady Heather accepts both positive
and negative delays, in fact cable delays are considered negative. I
didn't checked that the board retards the pulses with positive values,
I'll do it after the 1PPS mod but I think that this will work. I
believe that the elevation trick is not a good idea, it will ruin the
GPS receiver calculations, as you say it will work for 1 satellite view
and only when it is exactly overhead.
Best regards,
Ignacio
El 13/06/2015 a las 1:14, Ed Armstrong wrote:
Ignacio, I would very much appreciate a copy of whatever schematics
you have, even if it is not guaranteed to be 100% accurate
I agree with you that the 9.9804 Mhz is basically useless, while the
even second pulse is merely almost useless. However, as you have
apparently looked the board over more carefully than me, you probably
already understand why I did it the way I did. The location of the two
output circuits were very easy to find, the path from the connector to
them is quite distinctive. I just needed to find out where the signal
got into the output circuit from, and when I flipped the board over,
the trace bringing in the even second pulse was extremely obvious.
There was no obvious trace for the 9.9804, and I didn't feel like
probing all over the place and looking up a lot of chip numbers to try
to figure out where it came from, as I have a very unsteady hand which
makes poking around in these closely spaced components an invitation
to disaster. So I just went with the obvious.
I found it interesting that the output circuit inverts the signal a
few times. I actually would have preferred to invert it, so that the
polarity was correct for a raspberry pie or a serial port under
Windows, but it appeared some of the traces to do so were hidden in
the layers of the board, and again the more I fool around the better
my chance of shorting something out and becoming very unhappy.
I will be anxious to hear how your version of the modification works
out, please do keep us posted.
I believe the antenna cable feed delay is going to work in the wrong
direction here, I also seem to recall reading somewhere that the
adjustment range may be limited. I did pretty much correct the offset
by manually setting my position about 75M higher than what the device
figured it to be, but I am concerned that would only be accurate for a
satellite directly overhead, and may cause other inaccuracies by
throwing off the geometry, especially for satellites close to the
horizon. Based on what I am currently seeing from the Pi, I think the
smart solution is to just ignore the offset altogether.
Ed
On 6/10/2015 11:30 AM, EB4APL wrote:
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
_______________________________________________
time-nuts mailing list -- time-nuts@febo.com
To unsubscribe, go to
https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.
_______________________________________________
time-nuts mailing list -- time-nuts@febo.com
To unsubscribe, go to
https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.
_______________________________________________
time-nuts mailing list -- time-nuts@febo.com
To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.
