Hi Summing a pair of DAC's and checking them with an ADC is one way to get the job done. It's been used quite a bit.
16 bit DAC's are sub $3 items these days with pretty good specs on the parts. A multi channel <1 ppm accurate 24 bit "DC" ADC is a fairly common part as well. Raw parts cost from Digikey for PIC and the rest of it (except reference) likely would be sub $20. If you have all the parts already I suppose it could be free. Even with a $50 charge for a quick turn PCB there's not a lot being spent for the ADC side of things. You can spend a *lot* on a reference. That's going to be true for any long term stable stand alone EFC drive setup. My guess is that reference noise on affordable parts may drive you to the big R/C's. Putting at least one channel of the ADC after the big R/C lets you get a handle on leakage issues. Bob -----Original Message----- From: [email protected] [mailto:[email protected]] On Behalf Of Bruce Griffiths Sent: Tuesday, June 29, 2010 5:32 AM To: Discussion of precise time and frequency measurement Subject: Re: [time-nuts] yet another GPSDO design, or so Attila Kinali wrote: > Moin, > > On Sat, 26 Jun 2010 21:14:02 EDT > [email protected] wrote: > > >> What you want is basically a Shera Board. That design has been around for >> quite some time and has served me very well. >> > Yes. The Shera Board and similar designs serve as an example for me. > > >> I have a total of six running >> including two controlling Rubidium. There are in my opinion a couple of >> problems: not every 4066 works on the design the 18 bit D/A is very hard to >> find and now expensive and the single step of the D/A is intended for a 1.7 >> E-13 frequency step. >> > Yes. My goal is to update the venerable 4066 with something more > modern and have components that are easy to get trough farnell, digikey, > mouser, and all the other distributors. Yes, 16bit D/A seems to > be the maximum that is currently available. It crossed my mind > to build a 24bit R-2R D/A using discrete components, but this might > have actually a worse performance than a off the shelf 16bit D/A. > (temperature drifft, resistor values missmatch, EMI, etc) > > > Attila Kinali > Its possible to build a 24 bit resolution D/A using a synchronously filtered PWM circuit. A pair of PWM outputs and a few relatively low precision resistors and capacitors together with a low noise low drift reference are required. The technique takes advantage of the fact that the required EFC voltage changes slowly and isnt updated at a highg rate. The synchronous filter technique eliminates the very long time constant RC filters required with an asynchronously filtered PWM waveform. Bruce _______________________________________________ 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. _______________________________________________ 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.
