Technique for joint balancing of IQ modulator-demodulator chains in wireless transmitters

This paper presents a technique to jointly balance an IQ modulator and demodulator chain in a wireless transmitter. A scheme is introduced for simultaneously extracting the amplitude and phase errors in these two cascaded RF devices based on the measurement of one or two calibrated delay lines depending on whether the delay line losses can be neglected. An analytic solution is obtained for the extraction of these parameters from the three measured K-matrices relating the modulator and demodulator in-phase and quadrature components. The algorithm was implemented in a FPGA testbed. It was first applied to behavioral IQ modulator and demodulator implemented with their K matrix and verified to extract the correct amplitude and phase errors. A final demonstration study was then carried out with an actual physical IQ modulator and demodulator operating at 3.5 GHz and correct results within the phase margin were obtained for these devices. This technique for balancing a modulator demodulator chain may find useful applications in the loop back chains for adaptive linearization.