Re: NEO series GPS modules

Andy G4JNT

The classic "Simple GPSDO" such as   and   generally run with a PLL time constant of a few tens of seconds; perhaps as much as a minute.  But this is not sufficient to smooth out the variations on the GPS timing signal itself, due to propagation anomalies.  

See   for a set of short term stability measurements I made on several reference types including various GPSDOs

To smooth out GPS's own frequency shift, which can amount to a couple of parts-per-billion easily,  you need to move to a time constant of tens of minutes to hours and that runs into a new problem altogether.

The simple short time constant approach allows a low cost TCXO to be used as the oscillator element since the loop is fast enough to track its drift and wobble due to thermal effects (no TCXO has perfect compensation, ~~1ppm is all they're specified to).   With a time constant of minutes, it is too easy for the TCXO to wobble faster than the loop can compensate and that ends up worse than a fast loop

So you now need an ovenned source that can maintain good performance over tens of minutes to hours - and once you've got that, you have something that's probably going to be good enough on its own, with a periodic calibration / check.  And then, of course, there is the lock up time.  If your GPSDO has a loop time constant, say, of 20 minutes, it could well take up 2 hours to lock from a cold switch on.  As a rule of thumb, 4 to 8 time constants are needed to reach fulll stability.  So unless you have an intelligent computer controlled PLL that can adaptively adjust bandwidth as lock is achieved any /P event will be well underway before it is locked.    So this sort of GPSDO should never be turned off, and in fact makes a very good fixed lab standard.

As really good lab standard, a GPS locked rubidium source can probably not be bettered for home use.  Rb gives you parts per trillion short term stability over hours, and GPS locking with a time constant of very many hours or days corrects Rb's inherent slight error.   Such a setup would probably hold its own against the older second hand Caesium source.

All of which brings in a third problem : how do you actually do a PLL with a time constant that long?   Analogue R/C is out of the question, and if you try to do it with D/A converters and / or DSP, there is the quantisation issue.  (One failing of the VE2ZAZ design)  The best D/A converters are only 16 bit, and you'll need a finer resolution than that or a very narrow adjustment range.  Pulse width mod couldy be made to work, but only with a PWM of very much higher resolution than the hardware ones provided on-board chips - which means doing it in software / firmware.  And that high a resolution means a fast clock.

It ain't simple, doing it properly!   But for practical purposes, you can get away with a lot less.

 Andy  G4JNT

On 26 July 2016 at 19:51, Neil neil@... [ukmicrowaves] <ukmicrowaves@...> wrote:

Andy, when you say"very" narrow loop bandwidth, are we talking time constants of tens of seconds, or even longer? 

I'm with you totally on the Rubidium front, I now run an Efratom Rb source as the primary standard, and just use my G3RUH GPSDO for verification.  I also use a couple of Morions for /P, but I would like to lock them to GPS just for the sheer hell of it.  No real justification for doing it, perhaps I'm turning into a time-geek.

Neil G4DBN

On 26/07/2016 17:16, Andy Talbot andy.g4jnt@... [ukmicrowaves] wrote:
Instead of this rush for GPSDO based references with dubious short term stability, why not go for one of the several really good 10MHz OCXOs that have appeared recently or even a Rubidium source.   There are several Morion types out there, all capable of resetting to within a few parts per billion after a quite rapid warm up.

I have one in the 10GHz PortaBeacon that settles within a couple of tens of Hz (at 10GHz) within 5 minutes of switch on from cold - always.  And that module is an older generation OCXO  - later ones are better still.

Then use a basic GPS module with its 1 PPS output as the reference just for a frequency counter to confirm your calibration from time to time.

Best of all, get a Rubidium source - there are a few around on Ebay for £100 or so.   Not the cheapest option, but that will guarantee you sub PPB accuracy within less than 20 minutes of turn on.  

GPSDOs may be cheap, and although I've never tried these GPS modules being talked bout here, do get an uncomfortable feeling that unless used with a very narrow loop bandwidth locking up a reference, they won't be all that nice when multiplied up to the higher uave bands.

Unless someone knows better, and has done so ?

Andy  G4JNT


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