Yes, you are right: but actually I have a 2.5nS simple time interval counter in the FPGA and the only way to go beyond is the average. The sophisticated way would be to implement a tapped delay line or vernier delay line time-to-digital converter in a bigger FPGA than the XC3S50. And, yes, I have recently started my first GPS disciplined Rb with the same hardware. I have eliminated the fast and slow steps from the processing, using only the slowest one.
On Fri, Sep 14, 2012 at 9:50 PM, Bruce Griffiths <bruce.griffi...@xtra.co.nz > wrote: > Azelio Boriani wrote: > >> Also you need a super ultra fantastic voltage reference for a 32bit DAC. >> >> > Not really, the reference only needs to have low noise and good short term > stability. > Long term drift in the reference voltage will be corrected by the feedback > loop. > > Anyway, yes, in my GPSDO the controller has 3 levels: at startup is fast, >> then slow and then very slow. The levels trigger when the precision >> estimate is 10E-9 and 10E-11. If you have a resolution of 10nS then take >> 10 >> averages and your resolution will be 1nS and so on. >> > However the noise associated with the timing resolution doesn't average > down so quickly. > If such noise is random than at best it is reduced by SQRT(10) by > averaging 10 measurements. > There is no real substitute for lower noise, higher resolution > measurements. > > When I switch level, >> the number of averages is increased too but this leads to a slower DAC >> update rate. This is the problem: now I'm trying to figure out if the >> corrective action can be "predicted" (Kalman filtering) and applied at the >> same speed. >> >> > Bruce > > On Fri, Sep 14, 2012 at 8:52 PM, Chris Albertson >> <albertson.ch...@gmail.com>wrote: >> >> >> >>> On Fri, Sep 14, 2012 at 11:21 AM, Michael Tharp >>> <g...@partiallystapled.com> wrote: >>> >>> >>> >>>> Finally, do people think a 16 bit DAC is adequate or should I consider >>>> building a 32-bit one? I looked at a few designs when putting this >>>> >>>> >>> together >>> >>> >>>> but decided to keep it simple until things were up and running. >>>> >>>> >>> Having a 32-bit DAC would give you enough range so that you could drop >>> in any OCXO you might have. But if you can have trimmer resisters to >>> selected for your specif OCXO then 16-bits should be enough. If it >>> were me, I'd want the DAC steps to be smaller than what the phase >>> detector can measure. Said another way a 32-bit DAC might >>> eliminate the need for scale and offset trimmer resistors. >>> >>> Chris Albertson >>> Redondo Beach, California >>> >>> _______________________________________________ >>> time-nuts mailing list -- time-nuts@febo.com >>> To unsubscribe, go to >>> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >>> and follow the instructions there. >>> >>> >>> >> _______________________________________________ >> time-nuts mailing list -- time-nuts@febo.com >> To unsubscribe, go to >> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >> and follow the instructions there. >> >> >> > > > _______________________________________________ > time-nuts mailing list -- time-nuts@febo.com > To unsubscribe, go to > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. > _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.