Thanks.

kb8tq@n1k.org said:

If it slips, how long does it stay slipped?

I have plenty of old GPS units.  I'm a bit surprised I haven't noticed
something like that.  Maybe I'm not looking hard enough and/or in the right
place.

You also have the CPU crystal.  It's not good for long term stability, but
shouldn't have any troubles holding to a ms over several seconds.

--
These are my opinions.  I hate spam.

A nice post processing application such as RTKLIB will show any cycle
slips that your receiver might have encountered.

A good write up about cycle slips is available here:

https://rtklibexplorer.wordpress.com/2016/05/08/raw-data-collection-cycle-slips/
https://rtklibexplorer.wordpress.com/2016/05/08/raw-data-collection-cycle-slips/

Regards,

Chris

On 03/13/20 15:14:13, Hal Murray wrote:

Hi

If you always operate in fixed locations, your chances of seeing a slip
are not that high. The original post question and reference here were to
timing in a mobile situation.

Bob

On Fri, Mar 13, 2020 at 8:49 PM Bob kb8tq kb8tq@n1k.org wrote:

Thank you all for your inputs. Here is a little more data, prompted by
comments and questions. (Some of this I wasn't thinking about at first.)

I realized that, while I expect the majority of units to be stationary with
the ability to properly position an antenna, there is a chance that one or
more of the units may be mobile. Regardless of mobility, the purpose will
be time and frequency reference, not position. Does that change my choice
of device?

Some of the units may have access to the Internet and I could run NTP on
them but the majority will be using GPS as the single stand-alone source of
UTC. The goal is for all the units to be sync'd to UTC as they need to
perform repetitive functions concurrently.

The starting point for me was going to be the Ublox LEA-M8T because it is
current generation and Ublox offers a common footprint so that, if
production runs into the future, there will be newer modules that can be
dropped onto the same board. I want the second output to use as a 10MHz
reference. I have need for a 3e-7 accuracy 10MHz frequency reference in
some cases and that looked like it would do what is needed without going
full-on GPSDOCXO.

So is my first choice of the Ublox LEA-M8T a good one or should I be
looking at other units. While I don't want to spend $50(us) on a receiver,
if it does the best job, it will fit the budget and that extra timing
output I can use to generate 10MHz is quite attractive.

--

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

On 3/14/20 7:54 AM, Brian Lloyd wrote:

NTP provides advantages, as mentioned by Tim Shoppa, in that it also
does a "smoothing" of the time from your GPS.

Do you actually need 10MHz "control" or do you need "knowledge"?
If you count your clock with the 1pps, you get knowledge to 1E-7 (for
10MHz), but not necessarily control.
If you don't need good close in phase noise, or smooth ADEV statistics
you could probably use the count to drive a simple loop to steer a VCXO
every 1 second or every 10 seconds or something.

Conceiveably, one could also do something like continuously reprogram a
SiLabs part

Of course, cheap 10MHz VCXO (not VCOCXO) might get you there also.
Or use the Abracon
https://www.mouser.com/datasheet/2/3/ASG-C-9844.pdf
which has both a voltage and a programming input

$8 in single quantity from Mouser.

Hi

In order to get time, you first need location. Without location, there is no
way to correct for the time of flight of the signals.

In a mobile situation, timing can be tough. At some point a ZED-F9P or a
member of that family would do better if highly accurate mobile time is the
goal. Since your needs are modest, it may not be worth the cost.

There is still the issue of going under a bridge / into a “canyon” and
having the GPS stop due to lack of signal. This sort of dropout can
easily run into the “couple of minutes” range.

“Good antenna location” in this case is often not an easy thing to do.
Ideally you want a full sky view down to 10 or 20 degrees above the
horizon. Your typical “fast setup” location usually does not do this. With
an “impaired” antenna location, you can get dropouts based on the
limited amount of sky you can get at.

That output is likely to be very “dirty” as far as spurs and noise are concerned.
If you have any significant spectral purity needs … yikes.

Bob

On Sat, Mar 14, 2020 at 10:57 AM Bob kb8tq kb8tq@n1k.org wrote:

I understand. The question is, are the timing-oriented GPS modules going to
work in that application or should I opt for a position-oriented GPS and
accept lower accuracy of 1pps?

In a mobile situation, timing can be tough. At some point a ZED-F9P or a

And that is the question.

There is still the issue of going under a bridge / into a “canyon” and

Let me put it this way, if I am going under a bridge or in a tunnel, the
point will be moot.

“Good antenna location” in this case is often not an easy thing to do.

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.

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.

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."

--

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

Hi

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.

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.

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.

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.

Bob

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

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

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. ;-)

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.

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@lloyd.aero
+1.210.802-8FLY (1.210.802-8359)

Hi

It does and it doesn’t. You can not “count on” the TCXO to do nice things
in holdover.

So you need a second “something” to compare to that will let you make
this call. That or you shut down any time GPS drops out.

The time pulse, pps, whatever you call it is quantized to an internal free running
clock in the module. It comes out based on whatever edge happens to be
closest at the moment. Since the clock is free running you can have some
very odd behavior.

For accurate timing or a GPSDO, there is a “sawtooth” message that tells
you the offset of the edge from reality. That message comes out once a
second. If the edge you use comes out more often than that, you have
no feedback about where the edge is relative to what it actually should be.

The gotcha is that every application is different. Without a deep dive into the
entire design (and that’s not really what this list is for ) there is no way to start
sorting things out.  These days pretty much all the modules on the market
(that put out a pps) will easily do microsecond level timing.

Lots of fun !!!!

Bob