Conventional RC oscillators, though offer inexpensive components as resistors and capacitors, are not suitable for frequencies beyond few MHz. Here, a VCO based RC oscillator is proposed, where the output frequency can be programmed finely with digital codes over range of higher frequencies on the fly. In this architecture, divided output frequency of a VCO is compared precisely against an external RC time-constant at regular calibration interval and the VCO frequency is adjusted. The calibration rate can be adjusted by programmable choice of capacitor, to avoid any beat frequency with any system requirement. The circuit is designed and fabricated in a 0.18muM technology for low voltage operation.
