As I mentioned in a previous post, this Nixie retrofit is not about
"good enough", it is a learning experience for me to understand the ins
and outs of GPS based time, so it is going to do all kinds of things
that are not NEEDED but that is the fun of the project.
I'm also looking into designing my own patch antennas to get the best
sensitivity in the restricted confines of the block of wood, yet
something else to learn about.
That's what this is all about.
I yes I probably will do an FPGA controlled PLL for the fun of it (I
have done that before).
John S.
On 7/15/2016 9:54 AM, Bob Camp wrote:
Hi
On Jul 15, 2016, at 9:44 AM, David J Taylor <david-tay...@blueyonder.co.uk>
wrote:
Hi
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
signal
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 noise
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 Hz
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, anything
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.
Bob
===============================
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.
…. we had headed off into the “virtues” of PLL’s locking to 10 KHz ….
=====
To get a time resolution of 10 ms (yes 10X 1 ms), you don’t really need the
pps. The timing of
the serial string is probably “good enough”. That assumes you don’t have all
sorts of other
messages turned on as well. In the case of a wall clock, it’s not clear why
anything other
than a basic timing message would be enabled. A sub $10 module likely will do
everything
you need to do.
Bob
Cheers,
David
--
SatSignal Software - Quality software written to your requirements
Web: http://www.satsignal.eu
Email: david-tay...@blueyonder.co.uk
Twitter: @gm8arv
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