IMPROVED METHOD AND APPARATUS FOR TRACKING A RESONANT FREQUENCY

摘要

An arrangement for tracking the resonant frequency of one or more electrically resonant structures (11, 21) through a single channel comprises a variable frequency oscillator (13, 23) associated with the or each resonant structure (11, 21) which provides an excitation signal of a variable frequency encompassing the possible resonant frequency of the associated resonant structure. Coupling means (2) are provided which connect the or each variable frequency oscillator (13, 23) to said resonant structure(s). An I-mixer (15, 25) is provided for each oscillator (13, 23) which forms a synchronous detector, a first input (15a, 25a) of each I-mixer (15, 25) being connected to its associated oscillator (13, 23) and a second input (15b, 25b) being connected to the coupling device (2), the or each I-mixer (15, 25) mixing the excitation signal from the associated variable frequency oscillator (13, 23) with a response signal generated by the resonant structure(s) in response to the or each excitation signal. The output of the or each I-mixer (15, 25) is filtered to remove the sum products of the excitation and response signals, thereby leaving just an amplitude modulation component of the signal, which is then processed in a control loop to track the resonant frequency of the or each resonant structure.