a first crystal node for connecting to a first input to a crystal resonator;

a second crystal node for connecting to a second input to a crystal resonator;

a crystal driver coupled between the first and second crystal nodes, for energizing the crystal resonator;

a first frequency doubler, having a first differential input receiving the first crystal node and a second differential input receiving the second crystal node, the first frequency doubler generating a first doubled signal, the first doubled signal having a frequency substantially double a frequency of the crystal resonator;

an operational amplifier receiving the first doubled signal as a first input that is compared to a second input to generate a first buffered differential signal and a second buffered differential signal that have a phase difference of 180-degress; and

a second frequency doubler, having a first differential input receiving the first buffered differential signal and a second differential input receiving the second buffered differential signal, the second frequency doubler generating a final signal, the final signal having a frequency substantially four times the frequency of the crystal resonator,

whereby the final signal has a frequency that is substantially four times the frequency of the crystal resonator.