[time-nuts] GPS for Nixie Clock
kb8tq at n1k.org
Fri Jul 15 18:17:14 EDT 2016
If you are going to go “full boat” then you probably should get the sawtooth correction out of
the GPS and feed that into your control loop. You will need something you can run out at the
“few hundred seconds” sort of time constant.
> On Jul 15, 2016, at 5:25 PM, John Swenson <johnswenson1 at comcast.net> wrote:
> 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:
>>> On Jul 15, 2016, at 9:44 AM, David J Taylor <david-taylor at blueyonder.co.uk> wrote:
>>> 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.
>>> 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.
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>>> Email: david-taylor at blueyonder.co.uk
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