by John Dorighi and Rob Sleigh
An important consideration for improving the accuracy of time domain reflectometer (TDR) measurements is user calibration. A TDR user calibration is performed to correct for systematic instrument errors along with removing losses and reflections from cables, fixtures, and adapters. The measured system response is compared to the known response of a standard to determine the appropriate correction. Calibration ensures the most accurate measurements of a device under test (DUT). Traditionally, TDR calibration is performed by connecting a mechanical short and load at the measurement reference plane. Calibration using mechanical standards can be time consuming and error prone as channel count increases.
Electronic calibration, or E-Cal, is a solution to this problem. Electronic calibration was pioneered for Vector Network Analyzers (VNA) and has been in use for more than a decade to make calibration faster and simpler. E-Cal takes advantage of a single connection to the calibration module which contains various impedance states that can be automatically switched into the measurement path. This article will focus on the application of electronic calibration for TDR/TDT.