[time-nuts] GPS for Nixie Clock
kb8tq at n1k.org
Fri Jul 15 08:05:47 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 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.
> On Jul 15, 2016, at 1:19 AM, John Swenson <johnswenson1 at comcast.net> wrote:
> Why I was even thinking about it at all was that one of the models I looked at said the 10KHz was more accurate than the 1PPS, so I was kind of thinking about that. If the Ubloxes have 1PPS which are just as good then there is no reason to think about a 10KHz.
> John S.
> On 7/14/2016 9:45 PM, Chris Albertson wrote:
>> On Thu, Jul 14, 2016 at 1:34 AM, Hal Murray <hmurray at megapathdsl.net> wrote:
>>> If you are building a PLL, it's a lot easier to filter a 10KHz signal than a
>>> 1 Hz signal.
>> You are correct. But this guy is building a nixie tube clock. The
>> clock should increment the seconds at the tick of the UTC second.
>> There really is no way to do this without using the PPS. The serial
>> data is not aligned with the UTC "tick"
>> GPS receivers work just like the old telephone time service "At the
>> tone the time will be..." and then comes the leading edge of the 1PPS.
>> If he were building a frequency standard then, yes the 10KHz signal
>> would be the best one to use.
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