[time-nuts] ! PPS Source

[ed breya]

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