[time-nuts] NTG550AA 1 PPS mod
EB4APL
eb4apl at gmail.com
Wed Jun 10 15:30:18 UTC 2015
Hi Ed,
I am the one who discovered the 1PPS pulse while troubleshooting a
NTG550AA. Instead of reuse 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 wrapping wire to joint TP14 to
TP33 so the 1PPS will be at J5. I think that I will do the modification
this weekend.
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.
I had 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. Probably the easier way
to correct this is to use the command which sets the antenna cable delay
and compensate for the difference.
I don't have a full schematic, even I am not sure that the partial one
is 100% correct but I can send it to anyone who wants it.
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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