Digital Compensation of Frequency Dependent Imperfections in Direct Conversion I-Q Modulators

In a direct-conversion transmitter system, the non-ideal transfer characteristics of analog subsystems in the I and Q channels adversely affect the performance of the system. The study of static errors, including gain imbalance, phase errors, and DC offsets has received considerable attention in the literature. Much less attention has been paid to the effect of frequency-dependent imperfections that arise in the analog reconstruction filters commonly employed in direct-conversion structures. This paper presents results of research into robust methods for automatic digital compensation for these non ideal frequency-dependent characteristics. The design and implementation of FIR and eigenfilter structures to achieve compensation are presented. As a specific application the paper considers a direct-conversion system used to generate continuous-phase frequency-shift keying (CPFSK) signals. The performance achievable using the proposed digital compensation structures is presented, including measurement results taken on a DSP-based modulator platform.