[time-nuts] NTGS50AA 1 PPS mod
eb4apl at gmail.com
Sat Jun 13 13:56:25 EDT 2015
I just finish the mod. It was easy, I cut the trace between TP14 and
U405-6 and soldered a wire between TP14 and TP33. Now I have a pretty 1
PPS on J5, the old 9.8304 MHz output. The signal has 0-5 V levels,
normally high with a 10 us pulse going down. In my unit this pulse
leads the even second pulse by 539 ns. I will check if the Lady Heather
command for compensating the cable length can be used to move this if
somebody needs a more accurate "epoch second". I have to use the 1PPS
from my FE5680A as a reference but now it is disconnected.
I have made a picture of the mod and I'll include it with my partial
schematic (I made some advances there) and the list of the TP signals
that I'm preparing for upload.
I have checked that now I have also 4 additional 1 PPS outputs in the
110 pin connector J2. They are in the pins previously used by the
SYS_CLK signal. They are differential LVDS as most of the signals on
El 13/06/2015 a las 1:14, Ed Armstrong escribió:
> 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.
> 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.
>> 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.
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