[time-nuts] GPS module recommendation for Pi timing

[Brian Lloyd]

On Sat, Mar 14, 2020 at 12:52 PM Bob kb8tq <kb8tq at n1k.org> wrote:

> > I understand completely. Still, I have to come up with a
> reasonably-priced
> > source of time and it does seem like GPS is the right answer. I can live
> > with units going off-line periodically due to insufficient timing
> accuracy,
> > but I cannot have them drifting away from correct time.
>
> Only you can evaluate how important this or that aspect of the design is.
> If *any* timing drift past 1 ms becomes an issue, some sort of TCXO based
> GPSDO may be the real answer.
>

In reading the data sheet for the LEA-M8T it does have a TCXO for the
timing outputs.


> With a bare crystal, you could easily have a 10’s of ppm sort of error. If
> you do, then any outage over about 100 seconds will hit your limit.
>

Yes, and I just need to know that I am out-of-spec so I can turn off
participation by that device until it has good timing again. And if I have
to end up disciplining a local TCXO then I can do that too, and then I use
a cheaper GPS module. Ah, design decisions. Where would we engineers be if
we weren't making design decisions compromises. ;-)


> > The signal will be squared and used as a clock for something else.
> > Duty-cycle is flexible. As long as the rising edge is clean and there
> isn't
> > too much jitter, I can live with it. I have no erroneous notion that I am
> > getting a clean sine wave or a symmetrical square wave. Also, this is in
> > the "nice to have" category and I can do away with it and use a GPS
> module
> > that only has 1pps.
>
> The output will have jitter at the same level as the time pulse. If it’s a
> 20 ns
> time pulse pre-sawtooth, then the 10 MHz will also hop by 20 ns.
>

Yes, so what happens depends on whether it is phase error or phase jitter,
and then what the device wanting the 10MHz reference does with it. Since
that is part of this that I don't have control over, I am going to have to
get more information. But if that other time-pulse can be used as a 10MHz
reference that is used to discipline another oscillator rather than being
used directly, it should be OK. I just need more information on this part
of it. I just don't know how clean that output from the Ublox is.


>
> >
> > So I am still hoping that someone will say, "The U-Blox LEA-M8T is a
> pretty
> > good choice but for what you are talking about, you might want to look at
> > the XYZ module as well."
>
> There are maybe another two dozen modules out there that might work and
> cost the same or less. Buying a few of this and that / trying them out is
> about
> the only way to know if they are “good enough” for all the details of your
> application.
>
> The bigger issue is - how much can you invest to dig through a pile of
> modules?
> The simple answer is indeed to just pick one and accept the cost impact.
>

That is indeed the question. Given the experience level here I was hoping
that others had run into similar issues and might have more information to
share with me, or even point me to something that would works as well or
better than the Ublox.

Thanks! I appreciate your input.

-- 



Brian Lloyd
706 Flightline
Spring Branch, TX 78070
brian at lloyd.aero
+1.210.802-8FLY (1.210.802-8359)