Closed Loop Frequency Control for an Hyperthermia Magnetic Field Applicator

An automatic frequency control for a magnetic field applicator is presented. It operates at a frequency between 100kHz and 200 kHz. It is intended for a magnetic hyperthermia equipment used in oncology therapy experiments on small animals. The proposed scheme is based on a phase locked loop (PLL) which ensures operation at the natural frequency of a high Q resonant circuit, which includes a coil that produces the magnetic field. An analysis of the different blocks composing the system is presented along with a study of its stability and perturbation rejection features. Preliminary experimental results obtained on a low power prototype allowed to verify the theoretical models assumed for the different transfer functions the circuit involves. It presents a recovery time smaller than 10 ms for variations in the inductance value.