[time-nuts] GPS for Nixie Clock
David J Taylor
david-taylor at blueyonder.co.uk
Fri Jul 15 09:44:03 EDT 2016
Ok, I guess we need to go into this again:
All of the output signals generated by one of these cheap GPS modules
come from the internal TCXO on the module. All the signals.
None of the TCXO’s on any of these modules are tuned to match the GPS.
None of them, zero, not any.
All of the output signals from all of these modules are matched up to GPS
by guessing which clock edge to use.
The result for all of these modules and all of their output signals is a
with a *lot* of jitter.
All GPS based systems are limited by the noise of the GPS signal. It is
really dirty at short time intervals. The shorter the interval, the more
it has. Any signal that directly tracks GPS will be *very* dirty.
The only way to clean up GPS to make it useful as a frequency source is
with a very narrowband loop.
If you are implementing a < 0.01 Hz wide loop, it is no harder to do at 1
than 10 KHz or 100 MHz. In many respects it is easier to do at 1 Hz.
If the objective is a time *display* that is read with a human eye,
under 1 ms is not of much use. Your eye can’t detect it. Getting to 1 ns
is a different task than getting to 1 ms. A Hydrogen Maser flywheel is
not needed as part of a basic wall clock design.
Lots of variables, but also lots of basic facts.
That seems a somewhat negative assessment, Bob. For time purposes (even to
within a microsecond), the PPS output from the ublox etc. modules is more
than good enough (for a Nixie clock and many more needs). Couple the PPS
with the time from the serial output in your micro and that's completely
adequate. TCXO not even needed.
Yes, frequency is a different matter.
SatSignal Software - Quality software written to your requirements
Email: david-taylor at blueyonder.co.uk
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