[time-nuts] ! PPS Source

[Bob kb8tq]

Hi

Here’s the “whole story”, sorry if it repeats things you already know …

All GPS modules that I have ever seen use a free running clock. The internal 
oscillator is *not* locked to GPS. When they want to generate a 1 pps output
they drop / add cycles from the the internal oscillator to get it “as close as 
possible”. That means that you will always have an error in the PPS. 

Since they *know* this is going on, many devices report this error on a 
second by second basis. Since the error looks like a sawtooth if you graph
it, this is often called “sawtooth correction”.  This correction also takes care
of “hanging bridges” where the sawtooth stays to one side or the other of 
“correct” for a long time.

Normally when feeding a PRS-10, the sawtooth correction is not used. That
results in a degraded pps accuracy. The best GPS module to use in this 
case is one with a very small sawtooth “window” ( = a fast internal clock). 
Right now, the Furuno parts are winning this particular race.

If you *do* use the sawtooth correction (possibly by feeding a variable 
delay line chip), then indeed the F9P and F9T will do a much better job.

Some numbers:

Sawtooth on some older modules can be out around +/- 20 ns On newer
parts it might be down around +/-10 ns. On the F9 parts it is +/-4 ns. The 
Furuno parts run half that. 

Corrected, on a modern part, and looking at second to second variation, 
you can get below 1 ns with various modules. On the F9’s you can get well
below 1 ns.

=====

All of that is looking at short term variation. Your Rb does not move much 
short term (unless the temperature changes …).  Its stability and aging likely
are quite good. 

GPS (as received / uncorrected ) swings around a bit during a normal day. 
Swings of 10 to 20 ns are pretty normal. > 50 ns is possible under odd 
conditions. That’s more than your Rb is likely to move around over a 4 to 12
hour period. 

If you “follow” GPS with your Rb through a conventional loop, you likely 
degrade the stability of the Rb. It takes a fairly fancy loop to do a good job
on an Rb.

Bob

> On Aug 12, 2020, at 11:44 PM, Joe Hobart <nova at npgcable.com> wrote:
> 
> I have been using 1 PPS from a Motorola M-12 timing module to steer a SRS
> PRS-10.  I recently heard that a U-Blox ZED F9P module receives both L1 and L2
> and can provide much improved positional accuracy.
> 
> Would better positions translate into a smoother 1 PPS?  Does anyone have
> experience with this U-Blox module?  Can this be set up with a fixed position as
> a timing module?
> 
> Is there a better source of 1 PPS at a reasonable cost?  The U-Blox is about $200.
> 
> Thanks,
> Joe, W7LUX
> 
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