[time-nuts] NTG550AA 1 PPS mod

Bob Camp kb8tq at n1k.org
Sat Jun 13 09:17:45 EDT 2015


Hi

> On Jun 12, 2015, at 7:14 PM, Ed Armstrong <eds_equipment at verizon.net> wrote:
> 
> Ignacio, I would very much appreciate a copy of whatever schematics you have, even if it is not guaranteed to be 100% accurate
> 
> I agree with you that the 9.9804 Mhz is basically useless, while the even second pulse is merely almost useless. However, as you have apparently looked the board over more carefully than me, you probably already understand why I did it the way I did. The location of the two output circuits were very easy to find, the path from the connector to them is quite distinctive. I just needed to find out where the signal got into the output circuit from, and when I flipped the board over, the trace bringing in the even second pulse was extremely obvious. There was no obvious trace for the 9.9804, and I didn't feel like probing all over the place and looking up a lot of chip numbers to try to figure out where it came from, as I have a very unsteady hand which makes poking around in these closely spaced components an invitation to disaster. So I just went with the obvious.
> 
> I found it interesting that the output circuit inverts the signal a few times. I actually would have preferred to invert it, so that the polarity was correct for a raspberry pie or a serial port under Windows, but it appeared some of the traces to do so were hidden in the layers of the board, and again the more I fool around the better my chance of shorting something out and becoming very unhappy.
> 
> I will be anxious to hear how your version of the modification works out, please do keep us posted.
> 
> I believe the antenna cable feed delay is going to work in the wrong direction here, I also seem to recall reading somewhere that the adjustment range may be limited. I did pretty much correct the offset by manually setting my position about 75M higher than what the device figured it to be,


Don’t do that :)

The cable delay has far more range than you need for this sort of correction. It’s the ideal way to move the pulse around. It also is quite happy to be either negative or positive, so it can do a pulse offset either ahead or behind GPS. 

Changing the position does not put in a fixed time offset. Instead it adds an error to (almost) every datapoint you get. Since you are dealing with a 3 dimensional space, the error may be either towards or away from a given satellite. Yes some of those sat’s are out of your view. In your case the error varies from zero to 250 ns depending on the view angle. 

The net result will be a time solution that swings around a lot. The GPSDO will either try to follow this solution (and swing the frequency) or it will look at the data and toss the device into holdover. Exactly what the threshold of “I don’t like this location” varies a lot from manufacturer to manufacturer. 

====

The very real question is still - which edge is correct? Checking against the output of something like a cheap LEA-5T board would give you a quick way to work out what’s what. (You could equally well use an Oncore or any of the other boards out there). 

Until you have the output calibrated against some sort of independent reference, I would not worry a lot about correcting out a few hundred ns error when doing NTP.  It’s small enough that measuring it directly inside NTP will just drive you crazy. 

Bob

> but I am concerned that would only be accurate for a satellite directly overhead, and may cause other inaccuracies by throwing off the geometry, especially for satellites close to the horizon. Based on what I am currently seeing from the Pi, I think the smart solution is to just ignore the offset altogether.
> 
> 
> Ed
> 
> On 6/10/2015 11:30 AM, EB4APL wrote:
>> 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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