Coupling-induced hysteresis in free-running oscillators

An investigation of coupling-induced hysteresis in free-running oscillators is presented, for possible application in sensors and RF identification (RFID). In its basic form, the phenomenon arises when an inductive element in an oscillator circuit gets magnetically coupled to another inductor in an external (passive) resonator. For a low mutual inductance or coupling, a small dip is observed in the oscillator amplitude curve versus a turning parameter. As demonstrated in this work, when the mutual inductance increases, the dip evolves into a hysteresis cycle. When coupled to several external resonators, the solution curve will also exhibit several hysteresis cycles about the corresponding resonance frequencies. Due to the complexity of the solution curves, a dedicated analysis methodology has been developed and tested on a prototype at 600 MHz, obtaining very good agreement between simulation and measurements.