Extending the dynamic range of RF/microwave intermodulation measurements by multiple-carrier cancellation

Abstract

Multi-tone intermodulation (IM) measurement of highly linear RF/microwave components (e.g., multi-carrier amplifiers, antennas for wireless applications, cables and coaxial connectors) often rises difficult dynamic-range problems because it involves measurement of weak IM products in the presence of strong excitation signals. This paper investigates a multiple-signal-cancellation method suitable for suppressing these unwanted signal components, thus, allowing IM measurement of highly linear devices. Instead of measuring a chosen device at one time, several similar devices (supposed to have electrical parameters close to each other) are measured simultaneously. All the devices under test (DUTs) are excited with the same signal amplitudes but initial phases for different devices are chosen so that unwanted carriers cancel out at a chosen output, while intermodulation components add in phase at other outputs of the power combiner. The scattering matrix of the power-combining/splitting networks have to have particular orthogonality properties. It is shown that the discrete Fourier transform (DFT) matrix meets the requirements for signal separation, thus Butler matrices, can be used for IM-measurement purposes. The method is suitable for non-linearity testing of highly frequency dependent devices (e.g., superconducting filters). Experimental measurements at 1.5 GHz with a system comprising four MMIC amplifiers are presented.