A UMTS upconversion mixer in 0.25 um CMOS

The design of a fully integrated upconversion mixer for use in a UMTS transmitter is presented. UMTS is the system which is developped for wireless transmission with high datarates. It uses Code Division Multiple Access to separate users of the same channel at the same time. The transmitted signal of one user has a nonconstant envelope due to filtering that is applied to limit the bandwidth. In the future higher Crest-factors are expected due to the demand for higher datarates. So a high linearity of the transmit chain is necessary to transmit the data undistorted. A high-level analysis is performed to recalculate the specifications for out-of-band emissions and in-band signal quality to more designoriented specifications for the upconversion mixer (distortion, LO-leakage, mismatch). A mixer topology is chosen for its excellent linearity. Several non-idealities of this topology are analysed in detail. Since the mixer is not able to drive the load by itself, a high-frequency buffer is required after mixing. A number of current-mirror stages yields current gain with a low distortion. In the design trade-offs are made between mixerand buffer-gain, the number of stages and their mirrorfactors. The most optimal trade-offs are obtained by use of a simulated annealing optimisation loop. By using the accurate mathematical models developed a fast and accurate optimization can be performed. The presented design uses a 3-stage current buffer, delivering −5dBm of output power to a 50Ω load with an IM3 ≤ −49dB. Keywords— RF CMOS, mixers, upconversion, UMTS, linearity