A Comparison of Approaches to Carrier Generation for Zigbee Transceivers

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