[time-nuts] Ublox neo-7M GPS

[Dennis Ferguson]

On 21 Aug, 2014, at 16:27 , Tony <tnuts at toneh.demon.co.uk> wrote:

> David,
> 
> No problem, its still set-up. As you'd expect its rock solid at 8MHz with no visible jitter.
> 
> Can you point me to the datasheet you're referring to? The MAX-7 and NEO-7 datasheets don't provide any information on clocking.
> 
> In the 'u-blox 7 Receiver Description Including Protocol Specification V14' the only clue as to the clocking characteristics is that the timepulse output must be configured with a minimum high or low time of 50ns or pulses may be lost. Make of that what you will!
> 
> Tony

It sounds like this part is similar to the LEA-6T (including the 48 MHz
reference oscillator).  This white paper has some information about the
frequency output of the latter:

   http://www.u-blox.com/images/downloads/Product_Docs/Timing_AppNote_%28GPS.G6-X-11007%29.pdf

See, e.g., figures 11 and 12.

I think the cleanliness of the 8 MHz phase is a little bit misleading,
however, since the clean part isn't actually 8 MHz.  It is instead the
frequency of the free running 48 MHz reference divided by 6, and it will
still be throwing in the occasional short or long cycle to correct the
48 MHz oscillator frequency error and make the long term average come
out at a true 8 Mhz.

In some sense this is the high frequency equivalent of the 1 PPS "hanging
bridge" case.  The output at any frequency has a phase error of +/- 10.5 ns
but at 8 MHz the phase error of the output changes very slowly, and can
hang near one of the extremes for long periods, so it requires very long
integration times to reduce that to zero.  If you were using this output
to drive a cleanup PLL (which I would call the "DO" in a GPSDO) I think
you would actually be better off using the output at 10 MHz since with
that integrating over just a few microseconds of the jitter reduces the
short term average phase error by a factor of 5, to +/- 2.1 ns, and an
"odder" divider might be better still.

Dennis Ferguson