Modeling of intermodulation in a loaded-line phase shifter based on a polynomial varactor model

Journal article, 2024

This paper investigates if there is an optimum design of loaded-line phase shifters with respect to phase shift/loss figure of merit (FOM) and linearity. The investigation was performed by comparing six loaded-line phase shifters that were implemented in printed circuit board (PCB) technology with shunt-loaded hyperabrupt varactor-diodes. It was demonstrated that the hyperabrupt varactor's C-V characteristics must be modeled with high accuracy to predict the nonlinear behavior. A polynomial varactor model was employed and experimentally validated. To extend the range of investigated parameter values, the extracted model was scaled and evaluated further in a circuit simulator. The investigation reveals that for a given varactor-capacitance, the phase shift/loss FOM is improved if the varactor-capacitance is evenly distributed and the unit cell length is much shorter than a quarter wavelength. The study demonstrates that the phase shift/loss depends mainly on the distribution of varactorcapacitance and Q factor. The intermodulation (IM) distortion is primarily proportional to the total varactor-capacitance per unit cell. The study also revealed that an increase in the varactor's Q factor results in higher IM. Therefore, it is a trade-off between low loss and low IM.