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Source match and load match
A TRL calibration
assmes
a perfectly balanced test set architecture as shown by the term
which represents both the forward source match (Esr) and reverse load match
(ELR),
and
by the
cz2
term which represents both the reverse source match
(Esu)
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 HP 8753E 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
(Esr
)
= reverse load match
(ELR)
=
~11
reverse source match
(Esn
)
= forward load match
(ELF)
=
~22
For a fixture,
TRL*
can eliminate the effects of the hxture’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
insnitely
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.
Improving Raw Source Match and Load Match For
TRL*/LRM*
Calibration
A technique that can be used to improve the raw test port mismatch is to add high quality
fixed attenuators. The effective match of the system is improved because the tied attenuators
usually have a return loss that is better than that of the network analyzer. Additionally, the
attenuators provide some isolation of reflected signals. The attenuators
also
help to minimize
the difference between the port source match and load match, making the error terms more
equivalent.
With the attenuators in place, the effective port match of the system is improved so that the
mismatch of the
fixture
transition itself dominates the measurement errors after a calibration.
Application and Operation Concepts
6-95