Implementation of automatic digital compensation in IQ modulators

We consider Digital Signal Processor (DSP) based IQ modulators generating Continuous Phase Frequency Shift Keying (CPFSK) signals. Departures from a flat-magnitude, linear phase frequency response in the pass bands of signal reconstruction filters in the / and Q channels cause ripple in the modulator output signal envelope. Amplitude Modulation (AM) in the signal envelope function produces undesirable sidelobes in the FSK signal spectrum when the signal passes through nonlinear elements in the transmission path. A structure is developed for digitally compensating for the magnitude and phase characteristics of the signal reconstruction filters. Optimum digital compensation filters are found using least squares (LS) techniques. We propose a method by which the optimum compensation filters can be estimated using test input signals thus allowing the compensation process to be automated. We describe the implementation of the automatic digital compensation technique on a DSP platform and present experimental results.