In this project a 16-bit PIC is used to build an accurate frequency reference synchronized to a GPS clock. This device can serve as an accurate frequency counter that can count to 50 MHz with 1 Hz accuracy. It can also be used to generate GPS-locked output signals as a frequency reference or as a component in a radio receiver. Finally, the ‘normal’ GPS information (time, location, velocity, etc.) can also be displayed. This project uses 2 UART’s, all of the 5 timers, 2 PWM outputs, and an external interrupt.
The key feature of this project is the two varactor diodes whose capacitance is controlled by a filtered PWM output. The frequency of the 10 MHz crystal can be adjusted +/- 20 ppm by changing the PWM duty cycle. A 32 bit counter counts the system clock for 1 second. An interrupt routine invoked by an external interrupt from the GPS starts a PID controller to adjust the clock rate.
© Michael Mullins
Project Documentation And Schematic (zip)