An equation error approach for the design of digital IIR compensation filters in IQ modulator

In IQ modulators generating CPFSK (continuous phase frequency shift keying) signals, imperfections in the analog reconstruction filter cause ripples in the modulator output signal envelope. This results in spreading of the transmitted signal spectrum into an adjacent channels and thus, failing to meet the requirements on adjacent channel emissions. In this paper, we propose a state-space technique to design IIR precompensation filters to compensate for the magnitude and phase characteristics of the two analog reconstruction filters using equation error formulation. Simulation and experimental results are presented to show the effectiveness of this technique. We also compared the results obtained with that of other IIR compensated scheme (Soh et al.,2001).