[time-nuts] Testing frequency using NTP Bruce GPS ps
XDE-L2G3 at myamail.com
Tue Oct 14 03:54:44 UTC 2008
>> So where did the 1ns granularity come in?
> For example, Motorola receivers output the sawtooth correction as
> an 8-bit signed binary field in the @@En/Hn TRAIM message. The
> range of said byte is -128 to +127; the scale, the granularity,
> the units of that field are 1 ns. Make sense now?
Thanks, but I still can't make the connection. I have studied every
site I can find on GPS architecture. Some are very good, but I can't
find anything that relates a 1 second GPS message to the 1PPS pulse.
I can understand how the sawtooth is generated from a crystal that
doesn't have a convenient division factor to get to 1 Hz. I believe
similar pulse-swallowing techniques are used in synthesizers to
generate arbitrary division ratios. Unfortunately these have poor
In any event, my system ignores all the timing and quantization
errors in the 1PPS signal. What I am really interested in is how the
GPS receiver can identify the correct time messages from many
different satellites at wildly different distances, with huge and
changing doppler shift, and old satellites constantly leaving and
new ones appearing. That is a complete mystery.
>> Thanks, already studied and analyzed. He uses an XOR phase
>> detector, but the funny hump at 1,000 seconds bothers me. I can't
>> see a good reason for that. The drop starting at 1e4 is
>> interesting, and it would have been nice to see what happens
>> further out.
> The hump around 1000 seconds is characteristic of quartz-based
> GPSDO. It is related to the "cross-over point" where the
> instability of the XO meets the stability of GPS on an ADEV plot
> and to the time-constant of the PLL.
OK, thanks. That seems similar to the phase noise shelf in PLL loops.
But why can't the GPS correct the oscillator faster? Why is the loop
time constant so long?
I think that has to do with the errors in the 1PPS output pulse,
such as sawtooth, quantization, and so on.
And that what I hope my system will fix.
> Here's another GPSDO related link for you to study.
Yes. Drool, drool:)
Did you know your page at
is referenced by Bob at "Rubidium and GPS DO From China" on eBay?
To see it, go to eBay and search
RUBIDIUM DATUM LPRO 10Mhz Oscillator longer Lamp life
The graph at the bottom of the page is what convinced me to get the
LPRO instead of the cheaper rubidium version on eBay. Your graph
shows the LPRO is just as good as the PRS10 for averaging times of
one second or longer. That is fantastic! Thanks very much.
BTW, were you going to send me the log20696.txt file shown on your
web page? Or maybe link to it so others can se it also?
Thanks for all your help.
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