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
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