[time-nuts] ! PPS Source

[Bob kb8tq]

Hi

The TBolt is a GPSDO rather than a GPS module. It’s a very different beast than a
ZED-F9P.  ZED-F9T, or the  M-12 the OP is using.

Bob

> On Aug 13, 2020, at 12:09 PM, David C. Partridge <david.partridge at perdrix.co.uk> wrote:
> 
> IIRC the Thunderbolt DOES lock its internal clock to the GPS
> 
> David
> -----Original Message-----
> From: time-nuts [mailto:time-nuts-bounces at lists.febo.com] On Behalf Of Bob kb8tq
> Sent: 13 August 2020 14:39
> To: Discussion of precise time and frequency measurement
> Subject: Re: [time-nuts] ! PPS Source
> 
> 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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