[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?
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
"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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