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

John Ackermann N8UR jra at febo.com
Wed Jun 10 08:30:33 EDT 2015

Hi Ed --

For standard PC hardware, 250ns is way under the interrupt granularity 
of the computer, and will never be noticed.  Some specialized 
configurations (https://www.febo.com/pages/soekris/) have timer 
resolution to a few hundred nanoseconds, but that takes hacking.

I know there have been discussions about the RPi as an ntp server, and I 
don't recall anyone talking about what a wizard device is was as a pure 
timekeeper.  It ultimately comes down to the interrupt resolution, and 
the short-term stability of the computer clock source.

So, (a) I don't think you don't need to worry, and (b) even if you do, 
you can insert a "fudge" factor into the ntp configuration to neutralize 
fixed offsets like this.


On 6/10/2015 12:30 AM, 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 ┬Ás. 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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