Measuring Time-Varying $I/Q$ Impairments in Digital Transmitters

Impairments affecting the baseband modulator of digital transmitters, which are commonly called $I/Q$ impairments, are responsible for the deviations of symbols from their original position on the $I/Q$ diagram. They cause a reduction in the noise margin and ultimately worsen the probability of error. This is why their measurement is so important in all stages in the life cycle of digital transmitters. This paper presents an original method for the evaluation of the $I/Q$ impairments based on a discrete extended Kalman filter (DEKF). In the same way as alternative solutions already presented in the literature, the method exhibits good performance in the presence of time-invariant impairments. Differently from them, it allows accurate measurements of time-varying impairments, thus making real-time tracking of their evolution feasible. After a few theoretical notes on the $I/Q$ impairments and DEKF, the operative stages of the proposed method are described in detail. Several experiments are then conducted on radio frequency signals to assess the performance of the method; the obtained results are given and discussed.