Second Order Distortion in CMOS Integrated Mixers

Understanding the even order distortion, its effects and its origin, is very important in direct conversion radio receivers. This project was focussed on second order distortion in down conversion mixers as that is the most critical block regarding the overall second order linearity of a direct conversion radio receiver. Furthermore, implementation in standard CMOS was considered as it has proven to be more difficult to realize high second order linearities than with a more expensive BiCMOS technology. Specification for the second order intermodulation intercept point (IIP2) was found from system level simulations for a direct conversion GSM receiver. After that the effect of three most important nonidealities giving rise to second order nonlinearity where studied and simple design equations relating device parameters to the target IIP2 value were derived. It was found that the balance of the differential input signal is critical whereas the matching of input transistors and the load resistors is less significant. Finally, the effect of RF-LO cross talk was studied. A methodology for characterizing a technology for non-systematic cross talk was developed and an equation relating the non-systematic cross talk and the IIP2 where derived. A set of test structures was characterized and it was found that if all systematic sources of cross-talk can be eliminated it is be possible to achieve, in a CMOS technology, very high values (+61dBm) of mixer IIP2. (Søren Laursen, Torben Larsen)