Cross-coupled CMOS voltage controlled oscillators operating in the X-band

Abstract

Voltage controlled oscillators (VCOs) are electronic devices whose oscillation frequencies can be tuned by applying an external control input. A widely preferred topology is the cross-coupled VCO topology, which offers easy implementation inside integrated circuits. VCO designers take certain performance metrics into account for their designs, with the most prominent ones being the frequency tuning range and the output phase noise. These two metrics often require trading off from one another; as introducing more networks for tunability increases the overall noise within the device. With the aim of observing this trade-off between the tuning range and phase noise, four VCOs have been designed and fabricated in a single die with a 0.18 µm CMOS process. They are designed to operate in the X-band, at almost the same oscillation frequencies, to allow for easier comparison. Each VCO in the IC offers either more tunability with more tuning circuits or better phase noise performance with simpler circuits. Measurement results verify this hypothesis; a decrease in output phase noise is observed in the tested VCOs that contained simpler tuning network. With center frequencies of oscillation at approximately 12 GHz in the VCOs, tuning ranges as high as 25% are achieved in the VCO with most tunability, while phase noises as low as -106 dBm/Hz (at a 1 MHz offset) were achieved in the one with no tunability.