Effect of cooling on the probe system sensitivity for low signal strength RFI problems

Only highly sensitive probe systems can detect the weak field strengths that cause radio-frequency-interference (RFI) problems typically found within cell phones. The sensitivity of the probe systems depends on the probe factor and on the noise floor. The effect of cooling by liquid nitrogen on the received signal strength and the noise floor of three resonant probe systems has been investigated. They operate at the GSM, GPS, and WiFi frequency bands. Cooling increases the Q-factor of these resonant probes, increases the received signal, and lowers the noise floor. The sensitivity of the system, defined as the signal strength at which the Signal-to-Noise Ratio is equal to 0 dB improves by 3-6 dB.

I. Introduction

High-frequency harmonics generated from the digital ICs and switched power supplies may couple into the antennas which are integrated into mobile systems such as cell phones. The noise of the harmonics is added to the natural noise floor of the receiver and overwhelms weak signals which should be received by the receiver [1], [2]. Near-field probes [3], [4] and scanning technique are effective tools to investigate noise field distribution of circuits at high frequency and to locate the source of the EMI problems at circuit and chip level [5], [6].