Fixed-Load SOLT: Polynomial vs Characterized Device (CD)
SOLT calibration requires accurate knowledge of the behavior of the standards; SOLT calibration is only as accurate as the definition of standards used. In practice, the standards are defined by a third degree polynomial function obtained by curve fitting technique which approximates the true behavior of the SHORT, OPEN and THRU standards. When the SOLT calibration is done using polynomial definition of standards, it is called Polynomial SOLT.
The reflection coefficient of the LOAD is taken to be equal to 0 at all frequencies, therefore the minimum discernable return loss is equal to that of the LOAD used for Polynomial SOLT calibration.
When each standard of the SOLT kit is individually and accurately characterized, the calibration is called Characterized SOLT. Each standard has its S-parameters measured at the factory, and unique coefficient files are used by each calibration kit during VNA calibration. Because each standard is individually characterized and no approximate models or assumption are used in this technique, the accuracy of calibration is enhanced.