Ed,
Good work digging deep into that. I remember hearing about someone
playing with the Oncore oscillator. It was Robin Giffard, one of the key
architects behind the hp SmartClock series (58503A, Z3801A, etc.). A
copy of his paper:
"Estimation of GPS Ionospheric Delay Using L1 Code and Carrier Phase
Observables"
https://apps.dtic.mil/sti/pdfs/ADA497270.pdf
You'll see the Motorola Oncore VP mentioned in several places. He used a
5061B and a simple M/N PLL to give 3590 / 188 MHz = 19.0957 MHz.
I don't recall any other papers describing a similar experiment. In that
era the Oncore VP was one of the favorite timing receivers. You would
think if there was any merit to the clock hack then lots of people or
products would do it. But AFAIK, none did, not even hp. All of these
GPSDO had microprocessors and TIC onboard so using the GPS receiver
generated sawtooth correction message is all they needed.
/tvb
On 8/17/2020 1:53 PM, ed breya wrote:
Thanks all, for the info on this issue. It does indeed look doable to
experiment with phase-locking the GPS RX module's internal clock to
the ultimate 10 MHz GPSDO output. That is, doable, but not necessarily
easy or without problems.
I dug up my old notes and info from about ten years ago, and found I
had studied it quite extensively, and had some various schemes
sketched out already - I forgot about all this. In the notes I found
some pages from US Patents 4,785,463, and 5,745,741, which are
pertinent to the Motorola Oncore RX units. I also found page 26 of a
document "Remote Frequency Calibration: The NIST Frequency Measurement
and Analysis Service," which talks about it too.
The internal clock appears to be 19.096 MHz nominal, +/- 2 ppm, from a
TCXO, that may or may not have voltage tuning too, depending on the
model. The '463 patent appears to be about the original overall
design, while the '741 is about reducing the sawtooth error by doing
coarse digital corrections in the counter system, while also fine
tuning the VCTXO, to get an integer clock frequency under all
conditions. It mentions the Oncore model as prior art, and the
resulting sawtooth error.
As I understand it, the overall process disclosed is to allow for the
TCXO to drift to any frequency in range, but automatically tweak it
slightly to make sure it's an integer (Hz ) value, and set the counter
system to divide by that same integer value, so the 1 PPS output is
consistent, without sawtooth error. So, if the clock is nearly exactly
right on, the counter divides by 19,096,000, and it figures out how to
fine tune the clock to keep it there. If the drift goes beyond the
fine range to say all the way to the upper stated limit of +38 Hz, the
counter is set to divide by 19,096,038, and the clock is again tweaked
to keep it close to that integer Hz. How it does all this is disclosed
in the patent, but I haven't studied it enough to say any more.
It looks like this improvement was in a later model, or was perhaps
never actually used, since this was around the time that Motorola was
departing the GPS business. I assume the older Oncores like mine do
not have any of this improvement, so are subject to both the clock
drift and the sawtooth. But, one thing I got from this, is that if the
nominal clock and divide numbers match, and are fixed at 19,096,000,
then replacing the original clock with a sufficiently clean
synthesized external clock should work too.
I have a number of possible options, depending on the actual original
TCXO. If it's also voltage-tuned, but that isn't utilized, then it can
stay, but needs circuit mods to release and access the tune signal.
The clock signal is accessible at the TCXO, or possibly less risky in
2X form 38.192 MHz from the downconverter IC. This would be the best
option, to make an external PLL to tweak it. BTW I have a spare GPS RX
unit - I would not risk taking the Z3801A out of commission and
messing around with it.
If the TCXO has no tuning ability, then a new clock signal needs to be
made. Of all the schemes I sketched out back then, the most
straightforward seems to be simply adding 4.096 MHz from a VCXO, PLLed
to 10 MHz, and 15 MHz derived directly from the 10. These can all be
scaled up in frequency in various arrangements, and use standard
binary frequency XOs. The next notch up for instance, is 8.192 MHz +
30 MHz giving twice the clock, and so forth. Unfortunately, this
method is additive in frequency, which I hate - I prefer to take the
difference of two much higher frequencies, which is so much easier to
filter. I don't yet see any ways to do a difference method without
using special frequencies, so for now I assume regular old standard XO
frequencies will have to do, and the filter designs will need to be
fancier. The PLLing seems to be straightforward. I can get a decent 16
kHz comparison frequency with simple integer dividing, like 4.096
MHz/256 = 10 MHz/625, but would like to get it as high as possible,
without resorting to fancier schemes. Fractional-N is not in the cards
here, I don't think.
It would be nice if for some reason a 19.096 MHz VCXO module - or even
just a crystal - was available for cheap, but I doubt such exists
outside of the Oncore line. Maybe a TCXO from a junker would do, but
again, it has to have voltage tuning too, and I don't know if any had
that. (One of my more far fetched schemes even pictured thermal tuning
- TEC heating and cooling the TCXO module - but imagine the nightmare
of characterizing the part and the dynamics, and the mechanical and
control loop issues.)
So anyway, I think I can do it with the additive VCXO combo, but doubt
I ever will - there are too many projects to worry about without
reviving this one, but it's fun to ponder.
BTW one last thing is that in my collection, I have parts of an
ancient (ca 1990), huge Trimble L1/L2 GPSDO. I was going to share some
info on it, but it will have to wait for another time. I find it very
interesting.
Ed
_______________________________________________
time-nuts mailing list -- [email protected]
To unsubscribe, go to
http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com
and follow the instructions there.
_______________________________________________
time-nuts mailing list -- [email protected]
To unsubscribe, go to
http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com
and follow the instructions there.
