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

David J Taylor david-taylor at blueyonder.co.uk
Mon Dec 8 10:12:22 EST 2014

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


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?

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Email: david-taylor at blueyonder.co.uk 

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