Vector Measurement of "Nonlinear Transfer Function"

A technique for the precise frequency-domain characterization of weakly non-linear RF/microwave networks is described. The method, which is capable of measuring the so-called Nonlinear (Volterra) Transfer Function, proves to be invaluable in designing high performance pre-distorters to linearize non-linear RF/microwave devices. The non-linearity under test (NLUT) is subjected to stimulus from several sinusoidal signals and a beat frequency is observed. The stimulus signals also feed a reference non-linearity which too generates a beat at the same frequency. The phases of these two signals are compared in a pair of coherent, tuned receivers, as in a Vector Network Analyzer. A calibration technique allows us to de-embed the effect of the linear networks preceding and following the NLUT and the reference non-linearity. The implementation is described in the context of a broadband (50-1000 MHz) non-linear pre-distorter for linearizing a Mach-Zender type optical modulator, used for transmitting multi-channel video (CATV) signals over optical fibers. As the technique is simple to set up and easily automated, it can be used in both engineering and production environments.