Shifting Reference Planes
Feedlines are required in many circuits to connect ports to the device under test (DUT). If the length of a transmission line is more than a few degrees relative to a wavelength, unwanted phase (and possibly loss) will be added to the results. If the impedance of the transmission line differs from the normalizing impedance of the S-parameters (usually 50 ohms), an additional error in the S‑Parameters results. Thus, if we are only interested in the behavior of the DUT, any “long” feedlines connecting the ports to the DUT should be negated during de-embedding. The process of negating lengths of transmission line during de-embedding is known as “shifting reference planes”.
Reference planes may be specified in the project editor for boxwall, and co-calibrated ports, but not for delta-gap and via ports. When your circuit contains a reference plane, and de-embedding is enabled, em automatically builds and analyzes the calibration standards necessary to de-embed the port and shift the reference plane by the specified length.
NOTE: Reference planes are not necessary for de-embedding. If you do not specify a reference plane in the project editor, the reference plane length defaults to zero. This means that em will not shift the reference plane for that port when de-embedding is enabled. However, em will negate the discontinuity for that port.