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
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