A Modified Technique for Characterization of Baluns Based on One-Port S-Parameter Measurements

We present a modified technique for characterization of a three-port test set utilized to connect balanced one-port infrared detectors to single-ended port of measurement instrumentation. The test set operating as a balun is dedicated to spectral measurements of the noise power density of photodiodes mounted inside TO-8 package. Its scattering matrix needs to be known to deembed the noise and impedance performances of the photodiodes. Due to nonstandard ports on the balanced side of balun, this matrix is determinated indirectly with a calibration based on the reflection coefficients measured at single-ended port, while changing load conditions on the both other ports. To this end, we employ Open, Short, and $50 \Omega$ Match loads, whose characteristics have been obtained with electromagnetic field simulations. We calibrate the test set using the calibration loads combined in seven different pairs. The calibration results agree well with characteristics calculated using circuit simulations on measured S-matrices of the test-set components.