Low-Overhead RF Impedance Measurement Using Periodic Structures

Due to the need for higher reliability and performance from RF circuits, multiport reflectometers are increasingly used as low-overhead impedance monitors. In this work, using periodic structures as multiports is proposed. Periodic structures impose a new constraint on the multiport theory and simplify it significantly. This simplification leads to closed-form solution for calibration and measurement procedures. The closed-form solution also shows that any arbitrary periodic structure will always have a unique solution for both the procedures. Therefore, the proposed technique does not rely on frequency-dependent behavior of devices, such as directivity and phase shift to measure impedance. This fact leads to increased bandwidth and simplified design procedure. In addition, the proposed multiport structures can be calibrated using fewer known loads than the existing multiport techniques. This fact, coupled with the closed-form solution, reduces the computation overhead and test time. The theory and its robustness against nonidealities, such as part-to-part variation, are verified with Monte Carlo simulations. A practical embodiment of the technique is demonstrated with EM simulations and hardware experiments. In this embodiment, the multiport structure is embedded into an LC matching network. Hardware experiments show that the embedded multiport structure can measure test loads with high accuracy from 1.5 to 3.5 GHz, without degrading matching network performance.