A New Electronically Controllable Tuner System for Precise Noise Figure Measurement

Stephan Biber¹, Sebastian Martius², Klaus Huber²

In order to achieve the minimum possible noise figure for a low-noise amplifier (preamplifier) such as they are used in local coils of MRI systems, it is necessary to know the optimum source impedance of the preamplifier. As the optimum source impedance of transistors is usually not specified for MR frequencies, we propose an automated system based on an electronically controlled tuner with fully automated calibration and measurement evaluation. With that system we get deeper insight in both the reproducibility of the noise parameters as well as the noise added by the preamplifier when the coil does not present the optimum source impedance to the preamp (varying load situation from patient to patient).