Characterizing the S-Parameters of 75Ω Circuits using 50Ω Lab Equipment

Abstract: RF engineers working with cable, terrestrial, or satellite TV applications are frequently required to make S-parameter measurements. Using a minimum loss pad to transform the conventional 50Ω test port impedance to the 75Ω device provides a cheap, easy way to get reasonable measurements.

For most general lab applications below 1GHz, a PCB-mounted minimum loss pad built from 1% 0402 or similar resistors offers a quick and easy means to test a 75Ω circuit with 50Ω lab equipment. In most cases, the only correction factor required is the insertion loss of the MLP – 5.7dB plus any addition connectors. Difficult calculations or even Smith Chart work is often not required to make basic S-Parameter measurements.