Hi
> On Jun 16, 2015, at 9:20 PM, EB4APL <[email protected]> wrote: > > Hi, > > I has been making some tests with the 1 PPS output and here are the results: > > Lady Heather cable delay commands works with both polarities, i.e. it can > advance or retard both 1/2 and 1 PPS signals, I used as reference and > external 1 PPS signal from a Rb oscillator. > The first impression is that the 1 PPS edge leads the 1/2 PPS about 550 ns > and this difference is consistent after some cable delay commands and antenna > disconnections, it is maintained during holdover. > Then I tried several power cycling and warm resets and this make an annoying > thing to appear: during the acquisition and phase locking the difference > jumped between 540 and 140 ns in 100 ns steps. This is due to the 1/2 PPS > synchronization with the SYS_CLOCK signal, so when the internal 1/2 PPS moved > back and forth until the system phase lock is obtained, the output jumps > between successive cycles of the SYS_CLOCK. The annoying thing is that its > final state is not always the same, the final difference can be any of the > mentioned steps, from 120ns to 550 ns and there is not guarantee which one is > obtained while in my board the 540 ns difference is the most common. This sort of thing is what I was afraid of when the signal first started being discussed. > I don't know yet it the 1 PPS is closer to the "epoch second" or it is the > 1/2 PPS, I have to hook up a GPS timing module and an antenna splitter and > see what happens. Anyway since Nortel specifies a tolerance of +/- 1 us of > the 1/2 PPS with respect to GPS even second, any of the seen values are > within specs but it is not very convenient for Time Nuts. It is still quite possible that the “right” signal is the one you have dug out of the board and not the 1/2 pps Bob > > Regards, > Ignacio > > El 14/06/2015 a las 20:35, EB4APL wrote: >> Even if I get a cell site I would not use it for a "private network", here >> all cell phones are GSM not CDMA. >> The only use for the 9.8304 MHz is as a master for deriving serial comm >> clocks (i.e. 9600 is 9.8304 / 1024) but I don't plan to became a "Serial >> Comm Time Nut" yet. ;-) >> >> Ignacio >> >> >> El 14/06/2015 a las 1:48, Bob Camp escribió: >>> Hi >>> >>> Of course tomorrow you will stumble into a “great deal” on a complete cell >>> site that needs a 9.8304 MHz clock :) >>> >>> ==== >>> >>> One thing to watch: >>> >>> The pps you now have may or may not be deterministic in its relation to the >>> every other second output. It also may or >>> may not be in a fixed relation to GPS. I would bet money that it *is* in a >>> fixed relation and that it’s actually better than >>> the other signal. Just because I believe it to be true does not make it >>> true. It needs to be checked against something else. >>> >>> Bob >>> >>> >>>> On Jun 13, 2015, at 1:56 PM, EB4APL <[email protected]> wrote: >>>> >>>> Hi, >>>> >>>> I just finish the mod. It was easy, I cut the trace between TP14 and >>>> U405-6 and soldered a wire between TP14 and TP33. Now I have a pretty 1 >>>> PPS on J5, the old 9.8304 MHz output. The signal has 0-5 V levels, >>>> normally high with a 10 us pulse going down. In my unit this pulse leads >>>> the even second pulse by 539 ns. I will check if the Lady Heather command >>>> for compensating the cable length can be used to move this if somebody >>>> needs a more accurate "epoch second". I have to use the 1PPS from my >>>> FE5680A as a reference but now it is disconnected. >>>> I have made a picture of the mod and I'll include it with my partial >>>> schematic (I made some advances there) and the list of the TP signals that >>>> I'm preparing for upload. >>>> I have checked that now I have also 4 additional 1 PPS outputs in the 110 >>>> pin connector J2. They are in the pins previously used by the SYS_CLK >>>> signal. They are differential LVDS as most of the signals on this >>>> interface. >>>> >>>> Regards, >>>> Ignacio >>>> >>>> >>>> El 13/06/2015 a las 1:14, Ed Armstrong escribió: >>>>> 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, 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 >>>>>>> >>>> _ >>> >> > > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
