Time Domain Substitution Vs. Port Extension Vs. Frequency Gated By Time

By Don Metzger, CTO, Constant Wave

A common measurement problem involving Vector Network Analyzers (VNA) involves projecting the measurement accuracy from a reference plane, established through calibration, to another location. Moving the reference plane may be necessary because it may not be convenient, or even possible, to perform a calibration at the desired location. This might result from a lack of appropriate calibration standards or, perhaps, from a lack of suitability of the desired location for the insertion of multiple calibration standards.

Two common solutions to this problem are Port Extension and Frequency Gated by Time.

When using Port Extension, the location of the reference plane is moved by mathematically removing a length of transmission line from the measured S-parameters. If the mathematics represents a lossless transmission line, only the phase of the S-parameters is affected. If the transmission line is lossy, both the magnitude and phase are affected. Some newer VNAs make setting the reference plane location easier by setting it at the location of a short circuit. As will be shown, Port Extension can work well, if the characteristics of the network being measured are appropriate. However, if the network is not well behaved in the appropriate ways, then Port Extension is not a good solution to this measurement problem.