user manual

Chapter 3 45
Test Set Cal and SelfCal
Test Set Cal and SelfCal: Theory of Operation
Test Set Cal and SelfCal: Theory of Operation
Adding a multiport test set to the analyzer degrades raw performance of the test
system as well as introduces drift. Test Set Cal provides the vector error correction
that allows the system to achieve good performance. A Test Set Cal is a calibration
that is performed on a regular, but infrequent basis. The results of the Test Set Cal
are used by the SelfCal feature to remove the drift associated with the hardware of
the test set and analyzer to return the multiport system to an accurately calibrated
state.
There are two steps to the Test Set Cal:
The first step requires you to connect calibration standards (from a calibration
kit) and your own through-cables to the calibration reference plane. The
analyzer measures the known reference standards and uses those measurements
to compute error correction coefficients.
The second step occurs once you are done measuring the calibration standards
and store the Test Set Cal data. The multiport test set contains electronically
switched open, short, load, and through standards behind each port. (See Figure
3-3. This illustration depicts a two-port test set. The same internal standards are
also included behind every port of your test set.) These standards are very stable,
yet their calibration definitions are not known like the mechanical standards.
During this second step, the analyzer makes a measurement of each internal
standard and then, using the error coefficients that were computed from the
measurements of the external standards, derives a very accurate calibration
definition for each internal standard. Once this has been done, the internal
standards act as “transfer standards” for future automatic calibrations called
SelfCals.