Analytical and Experimental Study of Tuning Range Limitation in mm-wave CMOS LC-VCOs

In this paper, we present an analytical model for predicting the tuning range of CMOS mm-wave LC voltage-controlled oscillators (LC-VCOs). A detailed analysis of the frequency dependent quality factor (Q) of the LC-tank is performed to characterize the tank loss. The frequency dependent Q is used to size the transconductance (g(m)) of the cross-coupled pair to satisfy the VCO startup condition. The relationship between the cross-coupled pair g(m) and the operating frequency is also derived. With the above relationships, the frequency dependent tuning range is further calculated and compared with simulation results. To verify the analysis, three CMOS mm-wave VCOs are fabricated in a 130 nm CMOS process. The measured tuning range of the 26 GHz, 34 GHz and 40 GHz VCO is 25%, 21% and 18%, respectively, which is consistent with the presented tuning range model.