[time-nuts] NTGS50AA 1 PPS mod

EB4APL 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 
this interface.


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.
> 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

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