[time-nuts] ! PPS Source

Mon Aug 17 22:41:58 UTC 2020

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
>
>
>
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