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.

I. Introduction

Recent trends in modern mobile computing and communication devices show integration of an increasing number of radio frequency bands and antennas for implementing wireless interfaces for communication and high-speed data-transfer capabilities. For example, a typical modern mobile phone contains various RF subsystems such as GSM, GPS, Wifi, NFC, and Bluetooth. High-speed digital circuits can generate high-frequency noise due to their switching nature. When the noise contains spectral components in the wireless bands, it becomes a potential noise source for the radio-frequency interference (RFI). On-chip dc–dc converters, high-speed memory clocks, I/O busses, and LCD clock harmonics are a few typical examples of such noise sources. Other trends show an increasing volume density in electronic products, which is calculated as the ratio of the volume of space taken up by electronic parts to the total available volume inside the electronic product. The combined effect of all these trends results in the placement of the potential noise sources and victim circuits in close proximity to each other. This has the potential to cause severe RFI issues such as receiver desensitization, noise coupling, antenna detuning, etc. Various types of techniques for investigating an RFI in mobile devices have been studied in [1]– [3]. Near-field scanning probes provide an effective approach to measure and quantify electromagnetic fields in the vicinity of the probes. RFI issues such as receiver desensitization require the measurement of the electromagnetic noise well below –100 dBm to identify potential noise sources (i.e., low trace currents lead to very low magnetic field strengths).