[time-nuts] time-nuts equipment verification from scratch (was: WTB: GPSDO)

Chris Albertson albertson.chris at gmail.com
Tue Mar 21 01:07:08 EDT 2017

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 at 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 at 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

More information about the time-nuts mailing list