Robustness issues in digital compensation for IQ modulators

A flexible approach to the implementation of a radio transmitter is to synthesise base-band I and Q signals using a DSP (digital signal processor) followed by a vector modulator to up-convert the base-band signals directly to RF. This approach can be limited, however, by analogue reconstruction filters that are necessary to remove digital image components from the baseband signal spectrum before transmission. Methods have been proposed to digitally compensate for these shortcomings. While these methods have been shown to be effective in improving modulator performance for a single realisation of the reconstruction filters, no study has been presented on the robustness of these methods to variations in the reconstruction filter characteristics. We present the experimental results obtained for a number of different realisations of the analogue reconstruction filters. Furthermore we show that in certain situations it is desirable to enforce an overall I and Q channel frequency response that has a different magnitude characteristic from that of the nominal reconstruction filter.