Precision Alignment of I/Q Detectors Using Fourier Transforms

Solid state microwave power devices used in phased array radars are approaching peak pulse powers that only allow characterization under pulsed conditions. This type measurement is necessary because of excessive thermal stress when the devices are operated in CW mode. Pulsed characteriztion is also desired if pulse width is less than the thermal time constant of the device. New techniques for measuring pulsed microwave S-parameters have become necessary as a result of this trend in GaAs MMIC and discrete GaAs FETs. This paper describes the coherent detection process used in a pulsed test set designed to characterize active phased array modules. Both the analytical and measurement approach to precision alignment of the I/Q detectors to achieve output errors of less than 0.1 dB and 0.1 degrees are detailed. The fundamental approach is to inject an offset frequency into the I/Q detector, digitize the results and perform FFTs on the complex time domain results yielding an image rejection value of the offset frequency. This image level is directly related to the phase and amplitude imbalance of the I/Q detector.