Converting S-Parameters from 50Ω to 75Ω Impedance

Abstract: Cable television integrated circuits are usually designed to interface with 75Ω input and output impedance, while most RF test equipment is 50Ω impedance. In most cases, minimum loss pads are used to convert the input and output impedance from 75Ω to 50Ω, and visa versa, for testing purposes. Although convenient, minimum loss pads should be avoided when measuring S-parameters. One easy way to perform such measurement is by treating the input and output impedance of the cable device as 50Ω, perform the test, and use the equations and Excel spreadsheet presented in this application note to convert the 50Ω-referenced S-parameters back to 75Ω.