Automated Preamplifier Noise Parameter Measurement System Using a Combination Analyzer

Introduction Array coils for MRI use highly reflective, low-noise preamplifiers to boost signal levels while minimizing, respectively, inductive coupling [1] and added noise. The noise factor [2], i.e., the loss of SNR due to noise added by the preamp (or noise figure, NF, when expressed in dB) depends on the admittance seen by the preamplifier's input, Ys = Gs + iBs, according to the expression [3] | | s opt s n min G Y Y R + F = F / 2 − , where noise parameters Fmin, Yopt, and Rn are, respectively, the minimum noise factor, the optimal source admittance at which this minimum occurs, and the equivalent noise (or correlation) resistance that describes how quickly NF rises away from Yopt. These parameters are needed to design optimal noise matching networks between coil and preamp, but manufacturers often do not supply this information at MRI frequencies and/or at impedances other than the standard 50 Ω. Measuring noise parameters is also needed to identify inter-device variability for quality control. Accurate noise parameter measurement requires multiple measurements of NF at various values of Ys, as well as corrections for reflections of noise power at the preamplifier's input and other stages in the measurement system. The latter is a significant effect in highly reflective array coil preamplifiers used for preamp decoupling [1], because noise reflected back to the standard 50 Ω noise sources used by NF analyzers leads to incorrect NF readings. High frequencies (> 1 GHz) or high cost (> $100k) of commercial noise parameter measurement systems (for the telecom industry) are unsuitable for most MR laboratories. We present an automated system to measure noise parameters based on a common combination vector network analyzer (VNA) / spectrum analyzer (SA) (4396B, Agilent, USA) and the LabVIEW software environment (National Instruments, USA). Methods A standard PC running LabVIEW was interfaced to the 4396B and to a calibrated 50 Ω noise source (NW1M500-6-CS, NoiseWave, USA) whose output can be set to either room temperature (“cold”) or one that is much larger (“hot”, nominal excess noise ratio ENR = 6 dB). A low-noise preamplifier (Agilent 8447D) is used to boost signal strength and reduce the system’s own noise factor (F2). This setup (Fig. 1) allows a “Y-factor” measurement [3] to be performed at 801 frequency points, 10 averages in approximately 25 s by taking the ratio of hot (Nh) and cold (Nc) noise powers measured at the output of the device under test (DUT): ( ) 1 / / − c h N N ENR = F . Typical spectrum resolution and