A 10mW 37.8GHz current-redistribution BiCMOS VCO with an average FOMT of −193.5dBc/Hz

Abstract

The continued scaling of digital CMOS technology has enabled mm-Wave VCOs with record figures of merit [1-5]. This is mainly driven by the increase in cutoff frequency and decrease in power consumption brought by lower supply voltages. However, at mm-Wave, challenges such as low Q-factor of the tuning varactors and switched capacitors result in a sharp degradation in the resonator Q. For an NMOS LC-VCO (Fig. 8.8.1), a large bias current and high transconductance (gm) are needed to maintain a given oscillation amplitude and to satisfy the startup condition. Since gm has a weak dependency on current in strong inversion, it can primarily be increased by enlarging the device width, W1, as illustrated in Fig. 8.8.2. Further degradation in the device gm is experienced when the VCO operates near the transistor cutoff frequency, necessitating an even larger W1 (Fig. 8.8.2). This results in a large fixed capacitance Cfix1 and hence a limited VCO tuning range (TR) [1,2]. It can also be shown that for the same bias current (i.e. output swing), increasing W1 comes at the expense of large thermal (1/f2) noise. This can be illustrated by examining the excess noise factor F, defined as the ratio between the transistors' switching noise and the tank resistor noise [6]. As depicted in Fig. 8.8.2, an extra 5dB of 1/f2 noise is added to the VCO output when the transistor W1 is increased from 20μm to 60μm, which is required to meet a 2× startup margin. Moreover, increasing W1 leads to a higher contribution of 1/f3 noise from up-converted 1/f noise [7].