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

Bob Camp kb8tq at n1k.org
Wed Jun 10 06:58:36 EDT 2015



At best your NTP setup will be good to a few microseconds. By the time you get the time delivered (even over a LAN) it will be good to a few hundred microseconds at best. The ~ 250 ns error should not be a big deal. 

This being Time Nuts, you might take a look at the options on NTP and the drivers. I believe you will find an “offset” capability buried down in there. It’s most commonly used for radio (WWVB etc) based clocks. (They need to be corrected for propagation). Based on observing it’s use on public servers - it’s pretty easy to get the setting backwards. 

Of course it does beg the question of “which pulse is correct?”. Without further research, (and more timing gear :) there’s no real way to be sure that the even pps is right ….


> On Jun 10, 2015, at 12:30 AM, Ed Armstrong <eds_equipment at verizon.net> 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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