An Ultra-Compact, Linearly-Controlled Variable Phase Shifter Designed With a Novel RC Poly-Phase Filter

This paper proposes a new vector-sum type variable-phase shifter (VPS) topology for predistorting the phase of a modulated signal for an analog-predistortion power amplifier system. It has a continuous linear-in-degree control curve over a 90° phase-control range and has the smallest size among all those proposed CMOS works. The phase shifter utilizes an improved RC poly-phase filter to generate in-phase and quadrature-phase vectors. It uses fewer RC components but has a wider phase-splitting bandwidth than traditional RC filters, reducing the loss and size of the overall VPS. Specially-designed control circuits give the shifter a linear phase-control capability, minimizing the gain variation over the phase-control range. The phase shifter, optimized for WCDMA applications, has been fabricated in a standard 0.18-μm CMOS process. The area of the phase shifter core is 0.063 mm
²
. The measured operation frequency is from 1 to 2.1 GHz, which is an overlap of its 3-dB cutoff frequency and bandwidth of a 90° phase-control range. Within the bandwidth, this phase shifter displays a linear control curve with phase errors of less than ±1° over a 70° tuning range, making it suitable for accurate AM-PM error compensation.