Method for Automatic Calibration of I/Q Amplitude Balance and Quadrature Skew Using GNU Radio

Abstract

Software Defined Radio (SDR) transmitters that employ digital phase modulation can have imperfections in the transmitted waveform that either distort the intended signal or introduce undesired spurious interference. In the case of zero-intermediate frequency transmission, these impairments include amplitude imbalance between the in-phase and quadrature components, quadrature skew in the vector modulator, and local oscillator feedthrough. In the case of heterodyne transmission, where the digital modulation is applied to a carrier offset before digital-to-analog conversion, an undesired image may be present at the vector modulator output. These transmitter impairments can be observed and measured directly with a vector signal analyzer (VSA), which can also allow the engineer to visualize the digital modulation in the form of constellation plots or eye diagrams. However, if VSA equipment is not available, a series of simple periodic test signals synthesized in the SDR transmitter can allow an engineer to measure the same imperfections directly with a simple spectrum analyzer. This technique can therefore be applied in GNU Radio flowgraphs. In addition, the test signal sequence can also be used to calibrate out the effects with the introduction of predistortion in the transmitter. This further implies that a calibration process can be automated. This paper illustrates the use of a GNU Radio flowgraph in measuring and cancelling out phase modulation imperfections. Emphasis is placed on binary phase shift keying (BPSK), quadrature phase shift keying (QPSK), and offset quadrature phase shift keying (OQPSK) signals. The appropriate test signals to use for each case are identified, and the theoretical spectrum for each signal is derived. Examples of imperfections and the effects they have on the test signal spectrum are shown and interpreted, and the spectral analysis technique measurements are then compared to VSA results.