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

Chris Albertson albertson.chris at gmail.com
Wed Jun 10 09:22:07 EDT 2015


For driving NTP..

1) NTP works in microseconds and your networked clients will see accuracy
in the range of a few milliseconds.

2) There is a way to fix this in NTP's configuration file, you can specify
what the delay is.

On Tue, Jun 9, 2015 at 9:30 PM, 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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-- 

Chris Albertson
Redondo Beach, California



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