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 (g <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">m</sub> ) are needed to maintain a given oscillation amplitude and to satisfy the startup condition. Since g <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">m</sub> has a weak dependency on current in strong inversion, it can primarily be increased by enlarging the device width, W <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sub> , as illustrated in Fig. 8.8.2. Further degradation in the device g <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">m</sub> is experienced when the VCO operates near the transistor cutoff frequency, necessitating an even larger W <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sub> (Fig. 8.8.2). This results in a large fixed capacitance C <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">fix1</sub> 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/f <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ) 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/f <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> 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 W <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sub> leads to a higher contribution of 1/f <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> noise from up-converted 1/f noise [7].