De-embedding and unterminating microwave fixtures with nonlinear least squares

A general method of characterizing microwave test fixtures for the purpose of determining the parameters of devices embedded in the fixture is discussed. The technique was used to investigate deembedding under the assumptions that all measurement errors are random and normally distributed and that the standards are distributed uniformly around the Smith chart. It was shown that for any given number of standards, the greatest accuracy under these assumptions is achieved by utilizing a large set of known reflective loads. When the propagation constant and the reflection coefficients of the standards are not known, then equal numbers of thru lines and reflective loads give the highest accuracy, although not as high as when the propagation constant and reflection coefficients are known. The accuracy of the technique was studied and compared with that of the common open-short-load (OSL) and thru-reflect-line methods. The OSL technique was found to be considerably less accurate than using sets of offset reflective loads