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

EB4APL eb4apl at gmail.com
Thu Jun 11 10:07:01 EDT 2015


Hi Ed,

I have started another thread under the name "NTG550AA 1 PPS mod" for 
finding the subject easier and I include here my thoughts about this 
modification.

I am the one who discovered the 1PPS pulse while troubleshooting a 
NTG550AA.  For me 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 PPS. Instead of removing 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 to 
joint TP14 to TP33 so the 1PPS will be at J5.  I think that I will do 
the modification this weekend and will post the results and pictures.

I have 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 but probably  this is 
overkill and an easier way to adjust this is to use the command which 
sets the antenna cable delay and compensate for the difference.

Checking the specs documentation of a very close cousin of this board, 
the GSBW50AA, I found the requirement for the even second pulse: "+/-1 
μs traceable to and synchronous with GPS Time Even_Second with at least 
one satellite in view".  In fact this is something not easy to measure 
unless you have a calibrated 1 PPS source.
Another spec states " The falling edge of Even_Second shall occur 0-5 ns 
after the falling edge of SYS_CLK." (the 9.8304 MHz signal that is used 
as the clock reference in the CDMA system).
This specs are the reason why the even second pulse is synchronized to 
the SYS_CLK as I said before.

So my opinion is that the difference that you measured is not relevant 
because we can not be sure about the even second accuracy and if we need 
to be sure of  the absolute time we will need to compare the 1 PPS 
output against a calibrated source, maybe another GPSDO that has been 
compared with a known standard and compensated for the antenna cable 
delay.  Of course for a NTP server you have more than enough.

As I said in the other post I have a partial schematic of the board, ask 
me if you want a copy.
Best regards,
Ignacio


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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