Hi
> On Mar 21, 2017, at 1:07 AM, Chris Albertson <[email protected]> > wrote: > > I built one of these using a PWM DAC also. The design was posted to this > list so I can't take credit for the idea. But we used two PWM output > pins. The PWM provides more voltage range than is needed by the OXO's > EFC. To the output was scaled by a voltage divider. This also scaled down > thew step size. The second PWN output was scale down even more, like > maybe 100X more. The two PWM outputs were added. One does course > adjustment the other fine. The software first sets the course PWM and > then the fine one takes over. > > But the PWM output was just run through an RC filter with a very long time > constants low pass filter with corner freq. < 1 Hz. The goal was to build > a VERY low cost GPSDO and adding a good external DAC would add to the cost. > > Someone here recently suggested that one could do as well by simply > adjusting a good oversized crystal with a screw driver as they could using > a simple GPSDO. Well, before building the GPSDO I tried keeping my OXO in > sync with my Tunderbolt using just a screw driver and a dual trace analog > tektronix scope. It is REALLY hard to do with a screw driver. You need a finer adjust pot on the EFC. It’s no different than the process you describe above with the PWM’s. With a PPS that is good to 10 ns, you can get to 0.1 ppb in 1000 seconds with margin. You will have a pretty good idea of what is going on in 100 seconds. If you are going to rig it up, a 20 turn wire wound pot with a dial on it is the high end approach. Set up the pot with a ~1x10^-8 full scale range. You then have roughly 5x10^-10 per turn. The cool part is that you can log the readings and work out what’s going on with the OCXO. Bob > Some tines > I'd think I had it right then I'd look at the scope after 30 minutes and > fine one sine wave had gained 1/4 cycle on the other. Lots of reason for > this, perhaps one of my voltage regulators are temperature sensitive, > "stiction" in the screw I was turning. Who knows. But my simple GPSDO > would notice the 1/4 cycle error and fix it automatically > > It real life for practical purposes I use the Rb clock, was lucky to get > one at the old $35 price. > > On Mon, Mar 20, 2017 at 4:43 PM, Mark Sims <[email protected]> wrote: > >> The "DAC" was PWM based, but used a separate voltage regulator for the >> "reference". I never tried it using the USB power as the reference. >> >> The OCXO (+board) uses less than 500 mA warming up (which it does rather >> quickly). It's in a small hermetic package about twice the size of a >> standard DIP-14 oscillator package. There was a Ebay seller several years >> ago offering them at $15 each or 10 for $100. >> >> The Chinese "Arduino" board (it's not really and Arduino, just a MEGA 328 >> and a proto area) has a micro-USB connector for power input but does not >> implement USB data. I used the processor serial port with a level shifter >> dongle. The firmware was a cheap and dirty hack and I didn't implement >> much in the way of control or monitoring... never got around to improving >> it. The project was basically "Hey, I forgot I had those parts... Hmmm, >> one could build a simple GPSDO... why not? >> >> ----------------- >> >>> Did you use the Arduino's PWM output plus a LPF for the DAC, or a >> separate >> DAC? If PWM, did you have problems with noise or sensitivity to the >> USB-provided supply voltage? >> _______________________________________________ >> 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. >> > > > > -- > > Chris Albertson > Redondo Beach, California > _______________________________________________ > 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.
