7-70
Operating Concepts
TRL*/LRM* Calibration (ES Models Only)
Figure 7-44 8-term TRL (or TRL*) Error Model and Generalized Coefficients
Source match and load match
A TRL calibration assumes a perfectly balanced test set architecture as shown by the term
which represents both the forward source match (E
SF
) and reverse load match (E
LR
), and
by the ε
22
term which represents both the reverse source match (E
SR
) and forward load
match (E
LF
). However, in any switching test set, the source and load match terms are not
equal because the transfer switch presents a different terminating impedance as it is
changed between port 1 and port 2.
Because the standard network analyzer is based on a three-sampler receiver architecture,
it is not possible to differentiate the source match from the load match terms. The
terminating impedance of the switch is assumed to be the same in either direction.
Therefore, the test port mismatch cannot be fully corrected. An assumption is made that:
forward source match (E
SF
) = reverse load match (E
LR
) = ε
11
reverse source match (E
SR
) = forward load match (E
LF
) = ε
22
For a fixture, TRL* can eliminate the effects of the fixture’s loss and length, but does not
completely remove the effects due to the mismatch of the fixture.
NOTE Because the TRL technique relies on the characteristic impedance of
transmission lines, the mathematically equivalent method LRM (for
line-reflect-match) may be substituted for TRL. Since a well matched
termination is, in essence, an infinitely long transmission line, it is well
suited for low (RF) frequency calibrations. Achieving a long line standard for
low frequencies is often times physically impossible.
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