In the previous Exercise, we utilized two different sources for cos(ωct), the HP signal generator and a crystal oscillator. The signal generator allows the selection of any carrier frequency, but is big and expensive. The crystal is small and inexpensive, but only provides a single channel.
The relaxation oscillators we studied in 242 are tuneable and easy to build, but have poor accuracy and stability. Resonant oscillators made with inductors and capacitors are more stable, but require but require careful calibration. Although it only provides a single frequency, the crystal oscillator is extremely accurate and stable.
We need a source of high-frequency sinusoids which is