Hi Backing off a bit, there are a *lot* of different OCXO’s out there. Some will hold a few ppb for months in terms of aging. Others will age that much in a day. Some will be good to < 0.01 ppb due to room temp changes. Others will move fractions of a ppm.
On top of that, some OCXO’s have (several) PPM of tune range on the EFC and others may have a hundredth of that range. In some cases the “tight range” lines up with a good stability part. In other cases … not so much. Trying to come up with *one* DAC setup that works for every OCXO out there is going to get into heavy duty overkill on a lot of those OCXO. All of this before we even get into things like noise out of the DAC. ===== In a lot of cases, some form of biassed attenuator between the EFC and the DAC is a pretty good idea. Assuming this is a basement project, tuning that up is not a big deal. Re-visiting the tuning in a year or five also should not be that big a deal. Indeed just how you do up an EFC attenuator is a bit of a can of worms, but there are ways to do it. You can save quite a bit on your DAC this way ….. (and improve performance …). Bob > On Jan 14, 2020, at 1:27 PM, Dan Kemppainen <d...@irtelemetrics.com> wrote: > > So, Just a few numbers to look at. > > I'm currently starting at a GPSDO output. The DAC is a PWM DAC with 20 bit > resolution. Note the previous replies in regards to the PWM. Also note, PWM > is used here as the DAC in the micro was so poor it was not usable to drive > the EFC lead. > > The DAC covers the whole tuning range of the OCXO. In 60 hours it's required > on average 0.61 counts per hour of correction for long term drift (temp, > aging, etc.). > > In reality, since this is using a GPS as a reference and due to changes in > temperature, the DAC has been 'yanking' the EFC to keep phase lock. That peak > to peak has been +/- 40 counts in 60 hours. Again, that's at 20 bits, with > about 6.4V full scale EFC range, and about .645 Hz/Volt. > > Nothing stellar here in terms of OCXO, GPS, GPS antenna location, etc. > Depending on your OCXO and source, 20 bits may not be enough. > > Hopefully this gives you some numbers that are meaningful in your search for > a DAC. > > Dan > > > > On 1/14/2020 12:00 PM, time-nuts-requ...@lists.febo.com wrote: >> Message: 3 >> Date: Mon, 13 Jan 2020 16:24:56 -0800 >> From: "Lifespeed"<lifesp...@claybuccellato.com> >> To:<time-nuts@lists.febo.com> >> Subject: [time-nuts] DAC for OCXO disciplining >> Message-ID:<009001d5ca71$0ccc52d0$2664f870$@claybuccellato.com> >> Content-Type: text/plain; charset="us-ascii" >> I have been looking into all-digital PLL designs to discipline an OCXO, the >> scope of which would of course include GPS, as well as other reference >> sources. A component of the block diagram would be the DAC converts the >> digital control loop output for application to the OCXO tune port. In order >> to use such a circuit to improve Allan deviation, rather than degrade it, >> concerns like step size, Differential Non-Linearity (DNL), and possibly to a >> lesser extent voltage noise, are concerns. Some thoughts that have occurred >> to me are coarse and fine DACs, possibly sigma-delta or pulse width >> modulation (PWM). Or some combination of all the above. >> Any suggestions on topologies and/or candidate parts? One possibility for >> fine tuning might be the AD5791. >> Lifespeed > > _______________________________________________ > time-nuts mailing list -- time-nuts@lists.febo.com > To unsubscribe, go to > http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com > and follow the instructions there. _______________________________________________ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.
