A linear digital VCO for Clock Data Recovery (CDR) applications

A new digital Voltage Controlled Oscillator (VCO) with the capability of fine tuning as well as coarse tuning controls is presented in this paper. Unlike conventional inverter-chain-based VCOs, the new design has more linear dependency of output frequency to input control voltage. The new VCO is a good candidate for modern Clock and Data Recovery (CDR) circuits because of this dual control. As a proof of concept, the functionality of the new VCO is verified in a CDR circuit used in a monolithic Serializer and Deserializer (SerDes) test chip fabricated in 0.5um ON Semiconductor process. The SerDes test chip was successfully tested at an operating frequency of 90MHz with a lock range of about 4.6 MHz due to the fine-tuning of the VCO. It is expected that using a more advanced process technology, a higher operating frequency can be easily achieved.