A Frequency Tunable High Sensitivity H-Field Probe Using Varactor Diodes and Parasitic Inductance

A frequency-tunable resonant magnetic field probe is designed for near-field scanning applications for the radio frequency interference studies. Tunable resonance is achieved by using a varactor diode providing the required capacitance and the parasitic inductance of a magnetic loop (i.e., a parallel LC circuit). An equivalent circuit model for the probe is described, analyzed, and used for designing the probe for achieving maximum sensitivity. The resonance frequency of the designed probe is tunable in the frequency range of 900-2260 MHz that covers multiple radio bands, such as the GSM900, UMTS, and GPS bands. The sensitivity of the probe at the resonance frequency is about 7-9 dB higher than that of an equivalently sized broadband magnetic field probe throughout the tunable frequency range. The measured frequency response and sensitivity over a microstrip trace using the fabricated probe shows good agreement with the simulated results of the equivalent circuit model and the full-wave simulation model.