SNR Analysis and Optimization in Near-Field Scanning and EMI Applications

In a near-field scanning system, each element of the measurement chain contributes to the thermal noise power density: probe, cables, amplifiers, and the measuring instrument. The signal-to-noise ratio (SNR) is strongly affected by the source output impedance, source temperature, the lossy transmission lines between probe and amplifiers, amplifier noise, amplifier temperature, and amplifier gain. By minimizing the loss between the probe and by using ultralow-noise amplifiers (noise figure (NF) < 0.5 dB), SNR improves by >10 dB, compared to a setup using a 1-m cable and a 3-dB NF amplifier. A resonant probe that is cooled with liquid nitrogen improves measurement SNR by an additional 10–12 dB, as compared to a broadband probe of similar loop size. To combine the advantages of a resonant probe, without sacrificing the ability to measure broadband, a proof of concept is demonstrated that uses a tunable resonant probe which is synchronized to the frequency sweep of the spectrum analyzer.