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

[Keith Loiselle]

I work with Said at Jackson Labs.  I've been reading the time-nuts
discussion for a few years, but rarely chime in.  I saw this discussion and
wanted to make a couple points.

* The LTE Lite time accuracy specification corresponds with the Skytraq GPS
receiver's specs page which I have attached.  The specification is for the
output directly from the GPS receiver available on the LTE Lite Eval
Board's JP1 connector pin 12.  This specification assumes optimal antenna
placement and thermal conditions, and position hold mode. It is also an RMS
(1-sigma) measurement not a peak-to-peak measurement.

* The GPSDO-generated 1PPS on the LTE Lite Eval Board's J1 connector has a
phase offset to the GPS raw 1PPS that is shown in the PJLTS message (2nd
field).  The GPSDO functions to drive this phase offset to zero.  But at a
given time--especially shortly after power up--the offset may 100 ns or
more.

Keith


Keith

On Mon, Dec 8, 2014 at 7:12 AM, David J Taylor <
david-taylor at blueyonder.co.uk> wrote:

> From: 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?
> []
> - Dave
> ===============================================
>
> Dave,
>
> My comparison is against a Rapco 1904M, which is another GPSDO.  That does
> agree on a causal measurement with a number of simple GPS/PPS units I have.
> A u-blox LEA-6T shows about 80 ns later than the 1904M, and a u-blox NEO-6M
> between 50 ns early and 200 ns late, both after being on for just a few
> minutes, and with no special care in antenna placing.
>
> Do you think that your measurement (~35 ns offset) is consistent with the
> LTE-Lite specification:
>
>  "1 PPS Timing Accuracy from GPS receiver"
>  "<8ns to UTC RMS (1-Sigma) GPS Locked"
>
> and the specification of the Thunderbolt?
>
> Cheers,
>
> David
> --
> SatSignal Software - Quality software written to your requirements
> Web: http://www.satsignal.eu
> Email: david-taylor at blueyonder.co.uk
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