[time-nuts] PPS for NTP Server - How Close Is "Good Enough"?

[Ed Armstrong]

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