Mixer linearisation for software defined radio applications

The inherent non-linearity of mixers is particularly acute in broadband receiver design for software defined radio (SDR) applications. Here, the receiver frontend 'sees' not only the wanted channel, but also a number of nearby signals. A conventional mixer will downconvert all of these received channels to IF, thus adding inband interference to the wanted channel. In this paper, known mixer linearisation schemes are explained and a new technique using frequency retranslation within a linearised mixer architecture is presented. Two-tone-test results from a prototype offered 33dB reduction in the distortion products and 22dB suppression of adjacent channel interference (ACI) for a pi/4-DQPSK modulated carrier. A theoretical analysis is also carried out to demonstrate the amplitude and phase matching requirements of the technique.