GPSDO Project Updates #1

In this video I go over some updates on the GPSDO project.

I switched to a simple design that still give us the desired control topology of the original idea.

The new design uses a phase comparator made by standard 74HC logic. The phase difference of the VCO and the GPS PPS is measured by counting pulses of a free running 48Mhz oscillator.

After each PPS rising-edge the micro-controller reads the phase difference stored in the counter register and runs the digital control loop.

In the basic tests I'm using simple PI loop and it's working pretty fine.

I also made a temperature sweep to capture the point of zero temperature coefficient. The heater was then adjusted to the temperature where the tempco of the VCO is zero. The temperature sweep approach works very well because it measures the temperature coefficient of the full circuit and not only the crystal.

48MHz frequency multiplier https://youtu.be/A9g3nspGRc8

PWM DAC ripple cancellation https://youtu.be/89XPmNW-rgg ... https://www.youtube.com/watch?v=Cft5E-T9vTY