Vector measurement of nonlinear transfer function

A technique for the precise frequency-domain characterization of weakly nonlinear 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 predistorters to linearize nonlinear RF/microwave devices. The nonlinearity under test (NLUT) is subjected to stimuli from several sinusoidal signals, and a beat frequency is observed. The stimulus signals also feed a reference nonlinearity 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 nonlinearity. The implementation is described in the context of a broadband (50-1000 MHz) nonlinear predistorter for linearizing a Mach-Zender type optical modulator, used for transmitting multichannel 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.<
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