Second-Order Distortion and Mixer Baseband Interface
The downconversion mixer practically limits the out-of-channel second-order distortion performance of a direct-conversion receiver. The use of balanced topologies is mandatory in order to reduce distortion by making the second-order distortion a common-mode signal. However, the unavoidable device mismatches convert part of the common-mode in-channel distortion to a differential signal. The IIP2 of a circuit can be improved by increasing linearity, i.e. reducing the magnitude of the common-mode second-order distortion component, or improving matching, which means that a smaller part of the common-mode distortion becomes a differential signal. At baseband, channel-select filtering, large devices, and wide-band negative feedbacks can be utilized to enhance linearity and matching. These methods are not applicable at the RF because of the high operation frequency. At the RF front-end, it may be difficult to reduce the second-order distortion to a sufficiently low level by enhancing linearity since this cannot be implemented at the cost of a degradation in the other performance parameters.