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?

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. 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 <holrum@hotmail.com> 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 -- time-nuts@febo.com > To unsubscribe, go to https://www.febo.com/cgi-bin/ > mailman/listinfo/time-nuts > and follow the instructions there. > -- Chris Albertson Redondo Beach, California

Hi > On Mar 21, 2017, at 1:07 AM, Chris Albertson <albertson.chris@gmail.com> 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 <holrum@hotmail.com> 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 -- time-nuts@febo.com >> 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 -- time-nuts@febo.com > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there.