Digital Compensation in IQ Modulators Using Adaptive FIR Filters

This paper discusses digital compensation for frequency-dependent transfer characteristics and implementationerrors in digital PAM/continuous-phase frequency-shift keying(CPFSK) quadrature modulators. Recently, several methods havebeen proposed to digitally compensate for the shortcomings ofthe analog reconstruction filters in IQ modulators. While thesemethods have shown to be effective, they result in filters with longcoefficients that are computationally demanding to implementon the DSP. Furthermore, the modulator needs to be taken offlinewhile the precompensation filters are updated to reflect thechanges in the I and Q channel characteristics. In this paper, adigital compensation method is proposed here using two adaptivefinite-impulse response filters to compensate for the magnitudeand phase characteristics of the analog reconstruction filters in theIQ modulator. The experimental results show that this techniqueis effective and lead to substantial improvement of the outputenvelope ripples.