On 21 Aug, 2014, at 16:27 , Tony <[email protected]> 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 _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
