What are "standard classes" in a calibration kit definition?

A standard class is a group of from zero to seven standards that are organized according to the calibration of the VNA error model.  The number of classes needed for a particular calibration type is equal to the number of error terms being calibrated.  For example, a one-port calibration has three error terms in its error model - directivity, source match, and reflection tracking.  This calibration would require three standard classes, which might be labeled OPEN, SHORT, and LOADS.  The OPEN class could contain a single open circuit standard, the SHORT class a short circuit standard, and the LOADS class might contain two load standards, one for low frequencies and one for high frequencies.

When the VNA is being calibrated, the order of measuring the standards in a class is not important; however, one class must be completed before going on to the next.  If two standards within a class overlap in frequency range, the last one measured will be used for the overlapping range.  It is not always necessary to measure every standard in a class.  In the example above, if the VNA was only being used at low frequencies, it would not be necessary to measure the high-frequency load in the LOADS class.

The classes needed for each calibration type are as follows:

• Response cal - uses the response class.
• Response & Isolation cal - uses the response & isolation and one isolation class.
• One-port cal - uses three reflection classes, either S11A, B, and C or S22A, B, and C.
• Enhanced Response cal – uses three reflection classes, two transmission classes, and one isolation class, all in either the forward or reverse direction.
• Full Two-port cal - uses the six reflection classes, four transmission classes (forward transmission, forward match, reverse transmission, reverse match), and two isolation classes.  Note that the 8753/8720 network analyzer models do not explicitly define the isolation classes in the cal kit; the isolation standards are assumed to be loads.
• TRL Two-port cal - uses the TRL thru, TRL reflect, TRL line, and two isolation classes.