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 <eb4...@gmail.com> 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 -- time-nuts@febo.com > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
