A Comparison of Approaches to Carrier Generation for Zigbee Transceivers

Two methods for generating in phase and quadrature local oscillator signals at 2.4 GHz for Zigbee transceivers are investigated. In one method, the output of a 4.8 GHz LC VCO is divided by two to obtain I and Q phases at 2.4 GHz. In another method, outputs of a four stage differential ring VCO at 1.2 GHz are appropriately multiplied to obtain I and Q phases at 2.4 GHz. These circuits are designed and laid out in a 0.18 mum CMOS process and they operate from a 1.8V power supply. The former architecture occupies 0.052 mm², consumes7.56 mW, and has a phase noise of -117 dBc/Hz at 3.5 MHz. The latter occupies 0.021 mm², consumes 9 mW, and has a phase noise of -97 dBc/Hz at 3.5 MHz. Temperature variations of the ring oscillator are minimized using a combination of constant current and constant g_m biasing.