A New CMOS Active Transformer QPSK Modulator With Optimal Bandwidth Control

This brief presents a new quadrature phase-shift keying (QPSK) modulator for Bluetooth applications with an optimal transaction bandwidth control. The modulator reduces the bandwidth of the modulated carrier by minimizing their transition sharpness. CMOS active transformers are developed and utilized in quadrature oscillator and multiplexer of the modulator to provide comparable phase noise performance without using spiral inductors and transformers. The performance of the modulator is assessed using a 1.6-GHz QPSK base-band modulator implemented in TSMC 0.18-mum 1.8-V CMOS technology and analyzed using SpectreRF from Cadence Design Systems with BSIM3v3 device models. The total transistor area and power consumption of the modulator are 2840 mum² and 30 mW, respectively. The phase noise of the quadrature oscillator is 110 dBc/Hz at 500-kHz frequency offset.

I. Introduction

Phase-Shift keying (PSK) is one of the most widely used constant envelope modulation schemes in wireless communications. Quadrature PSK (QPSK) in which the carrier generated from an oscillator is shifted in 4 increments of 90 degree each and binary PSK (BPSK) where the carrier is shifted in two increments of 180 degree each are two common PSK schemes. In QPSK each symbol represents two bits while in BPSK each symbol represents 1 bit. The phase of the carrier is usually selected by a multiplexer driven by an incoming digital stream [1], [2]. While providing improved power performance and removing the need for a base-band with digital–analog converter (DAC), this approach suffers from the drawback that the modulation transactions of the carrier may occur at any phase of the carrier [3]. Due to the existence of sharp transactions in the modulated carrier, the bandwidth of the modulated carrier becomes excessive. This severely complicates the design of downstream circuitry. Offset QPSK (OQPSK), a technique that adjusts the relative phase between symbol bits, was proposed in [4] to overcome these drawbacks. Although improving the bandwidth performance, this approach requires a larger base-band encoder with a DAC. Further to this minimum shift keying (MSK) is a special OQPSK technique in which the symbols are sinusoidally weighted at the encoder DAC to minimize the bandwidth of the output carrier [5].