A Calibration Method Based on Transfer Function Extraction for Cartesian Vector Modulator

Abstract

In this paper, a calibration approach based on transfer function extraction for the Cartesian vector modulator (VM) is presented. Three kinds of VM models-the ideal VM model, the frequencydependent VM model and the modified frequency-dependent VM model, are introduced in the proposed calibration approach. The calibration approach starts with an initialization of the transfer function of the modified frequency-dependent VM model. Then, the parameters of the transfer function are modified and extracted from the data of the measured transmission state (transmission amplitude and phase) of the actual VM by iteration, until the transmission state predicted by the extracted transfer function agrees well with the measured transmission state. Subsequently, the extracted transfer function of the modified frequencydependent VM model is capable of describing the transmission characteristics of the actual VM, and the calibrated baseband control voltages for the desired transmission amplitude and phase of the actual VM are able to be obtained by using the extracted transfer function. An actual VM is used as an example to verify this method. By adopting the proposed method, the maximum amplitude and phase errors at different complex gain setpoints are reduced to 0.05 dB and 0.3 • respectively after only two iteration steps. Since the actual VM is able to be accurately calibrated in only a few iteration steps, the results reveal that high accuracy and efficiency can be obtained in this calibration technique, which is well suited for applications involving high-accuracy calibration, real-time calibration and multichannel VM system calibration. INDEX TERMS Calibration, gain control, phase control, transfer function, vector modulator (VM).