[time-nuts] 10MHz LTE-Lite - PPS accuracy?

[Dave Martindale]

What is the source of the 1 PPS you are comparing against?

I compared my LTE-Lite to an old Thunderbolt (original model, single 24 V
input with internal DC to DC converters, Piezo oscillator).  At the time,
the Thunderbolt had been running for a few months, while the LTE-Lite had
been running for a week or so.  Antennas were sitting on the window ledge
of a west-facing window, so relatively poor sky coverage.  I connected the
PPS outputs from the two GPSDOs to two channels of a digital scope and left
it running in "accumulate" mode.  A couple of the resulting displays are
attached below (I hope).  Yellow trace is the Thunderbolt PPS, also the
trigger source.  The LTE-Lite is the cyan trace.  Each image shows signals
accumulated over a period of about 8-12 hours.

As you can see, the relative timing of the two 1 Hz signals wanders by
about +- 100 ns around a midpoint value, but at this midpoint the LTE-Lite
is around 50 ns later that the Thunderbolt.  (I call it a "midpoint"
because it's judged by eye as halfway between the two recorded extremes.  I
don't have a record of the individual measurements, so I can't calculate
mean or median).  The Thunderbolt's antenna cable is perhaps 10 feet
shorter than the LTE-Lite's, so that accounts for ~15 ns (Thunderbolt
antennas compensation is set to zero).

So, at my house, the LTE-Lite is about 50 ns late (or the TB is 50 ns
early).  That's one cycle of the LTE-Lite 20 MHz TCXO - coincidence?

I also have an old Garmin GPS-25 board.  This is a navigation GPS, without
timing features, but it does have a 1 PPS output.  I've included one
capture of GPS-25 vs. Thunderbolt.  The jitter is much worse; most (but not
all) traces are within +- 400 ns of the Thunderbolt (note the different
horizontal sweep).  And there is also an overall bias: the Garmin receiver
appears to be about 100 ns late on "average" compared to the TB.

Unfortunately, I don't have any other way to measure which GPSDO has the
more accurate PPS, and which one is responsible for most of the jitter.  (A
man with two GPSDOs never knows what time it is, precisely).

I do have a big old 5 MHz OCXO pulled from a Transit receiver which is
probably quite stable, but it is 0.2 Hz off nominal frequency and is not
adjustable.  Viewed on a scope alongside either GPSDO output, the 5 MHz
phase shifts by one cycle every 5 seconds, too fast to make any comparison
by eye of the stability of either GPSDO.

- Dave

On Mon, Dec 8, 2014 at 7:17 AM, David J Taylor <
david-taylor at blueyonder.co.uk> wrote:

> On the 10MHz LTE-Lite, how far out from true UTC would the PPS be expected
> to be?
>
> It seems to be about 200+ ns late on my unit, although it is much more
> stable than a typical GPS/PPS produces.
>
> Thanks,
> David
> --
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