Hi If you only run over 10% of the EFC range, you only gain 3 bits. If the objective is in the 22 bit vicinity, (maybe 20 maybe 22 …) you really don’t get enough bits at a 10% span. From a lot of years of playing with control loops, if you need 20 *good* bits, you better have a few more than that in the design …. Indeed there are converters out there with 1/8 LSB performance. There also are a lot of them with “guaranteed monotonic” as the main spec. In that case you may get 2 LSB of “jump” as you do this or that….
Indeed another alternative is to let the OCXO warm up for a month. Then adjust a pot to center things up. Run a “fine range” ADC to keep it happy. Come back in three to nine months and tweak the pot again. The main risk is a power outage and waiting a few weeks to get things back up and running again. A lot of this depends on how much of an EFC range you have and how much aging you expect. If you have 4 PPM of EFC and expect 1x10^-9 per month that gets you a pretty small range. If you have 5x10^-8 of EFC and expect 1x10^-9 per day, the entire EFC may not last you for very long at all. Another factor is temperature. Your OCXO may be happy at 1x10^-11 / C. If your control circuit is good at the 5x10^-10 / C level that may be ok or it may be a problem. Either way, your control range needs to accommodate both the OCXO and the rest of the circuit on top of the aging. Bob > On Nov 26, 2017, at 1:05 PM, Azelio Boriani <azelio.bori...@gmail.com> wrote: > > ...and what about shrinking the 16bit over the fraction of the EFC > range that, for example, the OCXO will be using for the next 5 years? > 16bit over 10V are as 20 (a little less, OK) over 1V, if I can use my > 16bit over 1V for the next 5 years, when the DAC will be near full > scale I can "trim" the aging. > > On Sun, Nov 26, 2017 at 6:25 PM, Bob kb8tq <kb...@n1k.org> wrote: >> Hi >> >> If you sum two DAC’s without any sort of feedback, you get problems when the >> “coarse” dac is changed. You have no way to know the step size of the coarse >> dac to (say) 20 bit precision. >> >> As an example : If you are after 20 “good” bits, you might overlap >> them at the 10 bit point on the coarse dac. That would give you 22 bits on >> the >> summed output. It would give you enough extra bits to take care of any odd >> things that might be going on. You only have 1/1024 of the total range before >> you must tune the coarse dac. Even with a good set of parts, you *will* be >> doing coarse tuning. >> >> Bob >> >>> On Nov 26, 2017, at 12:13 PM, Azelio Boriani <azelio.bori...@gmail.com> >>> wrote: >>> >>> Is summing a "fine tune" 16bit DAC and a "coarse tune" 16bit (or less) >>> DAC with an op-amp not good enough? >>> >>> On Sun, Nov 26, 2017 at 5:53 PM, Bob kb8tq <kb...@n1k.org> wrote: >>>> Hi >>>> >>>> Each time I’ve tried the method in the app note, there has been a tone in >>>> the output >>>> spectrum at the sample rate of the ADC. I’ve never found a way to do the >>>> grounding >>>> that eliminates it. The tone is large enough to show up as a spur on a >>>> “typical” OCXO >>>> when it goes into the EFC port. >>>> >>>> Bob >>>> >>>>> On Nov 26, 2017, at 8:56 AM, Ole Petter Rønningen <opronnin...@gmail.com> >>>>> wrote: >>>>> >>>>> I guess everyone has seen this, but Linear has a nice appnote «A >>>>> Standards Lab Grade 20-Bit DAC with 0.1ppm/°C Drift» >>>>> >>>>> http://cds.linear.com/docs/en/application-note/an86f.pdf >>>>> >>>>> Ole >>>>> >>>>>> 26. nov. 2017 kl. 13:50 skrev Magnus Danielson >>>>>> <mag...@rubidium.dyndns.org>: >>>>>> >>>>>> Hi >>>>>> >>>>>>> On 11/26/2017 02:26 PM, Attila Kinali wrote: >>>>>>> Though, if you have a decent 16bit DAC and want to get to 18bit, >>>>>>> that's fairly simple using delta-sigma modulation... if you can live >>>>>>> with a low pass fillter after the DAC. But the DNL will be the limiting >>>>>>> factor here (unless you use some special techniques) and the (absolute) >>>>>>> INL >>>>>>> will not get better, for obvious reasons. >>>>>> >>>>>> I needed 19 bit rather than 16 bit, so I implemented an interpolation >>>>>> scheme. A first degree sigma-delta would also be possible, but for low >>>>>> ratios what I did was more efficient. >>>>>> >>>>>> A first degree sigma-delta is fairly simple thought. >>>>>> >>>>>> The trick is that you want to push the noise high up so it becomes >>>>>> trivial to filter, then the filter will not be hard to design and won't >>>>>> be low enough to cause PLL instability and implementation troubles. >>>>>> >>>>>> Cheers, >>>>>> Magnus >>>>>> _______________________________________________ >>>>>> 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. >> >> _______________________________________________ >> 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.
