[time-nuts] NTG550AA 1 PPS mod

EB4APL eb4apl at gmail.com
Wed Jun 10 15:30:18 UTC 2015

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 
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.
> Ed

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