Sidelobe Levels of Intermodulation Distortion Due to Element Pattern Variations in Phased Array Receivers

Abstract

This article develops the theory to predict the sidelobe levels (SLLs) of intermodulation distortion in phased array receivers and validates the theory with Monte Carlo simulations and over-the-air measurements. It has previously been established in the literature that when intermodulation distortion is generated by signals incident on a phased array, the intermodulation distortion signal component has a linear phase relationship across the elements in the array such that the intermodulation distortion behaves as if it has an apparent angle of arrival. It is shown in this article that the SLLs associated with the intermodulation distortion signal components are significantly higher than the SLLs of the fundamental signals as a result of the error levels increasing through the nonlinear components. This is confirmed with a measurement campaign of 14 different test scenarios, in which the average SLLs of the intermodulation distortion were 7.6 dB higher than the average SLLs of the fundamental signals. When the receiver is beam steered to a direction away from the apparent angle of the intermodulation distortion, the predicted SLL can be used to calculate an effective third-order intercept of the receiver that takes into account the anticipated spatial rejection of the beamformer. This information helps predict the system performance of a phased array receiver in the presence of nearby interfering signals and can help determine receiver linearity requirements.