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

Bob Camp kb8tq at n1k.org
Mon Dec 8 17:54:11 EST 2014


If you read the NIST papers where they have looked at the PPS accuracy compared to UTC, the results are not all that good. The assumption that any one GSDO is “correct” compared to UTC is *not* a good one. The consistency of a GPSDO is quite good. That’s a very different thing than it’s accuracy (delta to UTC). In the case that absolute error relative to UTC is a requirement, you need a local UTC reference. The antenna delay setting is then used to “align” all of your GPSDO’s against your reference.  On many GPSDO’s the antenna delay adjustment is a 100 ns resolution sort of thing.

Again, it’s important to understand that these boxes were all made for cell service. That’s not an application where exact traceability to UTC is needed. Simply having all the sites run the same (consistent) GPSDO is perfectly adequate. If you have two brands of GPSDO, figure out the offset between them, still no need for “real” UTC. The “UTC” specs you see are one sigma bounds on the wander. Offset / centering of that peak are an unknown that is buried deep in the fine print. 


> On Dec 8, 2014, at 10:12 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
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> Email: david-taylor at blueyonder.co.uk 
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