Described herein is a passive wireless resonating non-linear sensor that is typically based on a ferroelectric varactor. The sensor replies its data at an intermodulation frequency when a reader device illuminates it at two closely-located frequencies. Furthermore, described herein is a general intermodulation read-out principle for passive wireless sensors. A sensor utilizes a mixing element, such as a varactor and it can be equipped with a capacitive, inductive, or resistive sensor element. When the sensor is illuminated with signals with two frequencies it replies the sensor data at an intermodulation frequency.