Atek HP 8753D, User Manual, Page: 354 / 677

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
"
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 dierent terminating impedance as it is changed between port 1 and
port 2.
Because the standard HP 8753D network analyzer is based on a three-sampler receiver
architecture , it is not possible to dierentiate 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
) = "
11
reverse source match (E
SR
) = forward load match (E
LF
) = "
22
F or a xture , TRL* can eliminate the eects of the xture's loss and length, but does not
completely remove the eects due to the mismatch of the xture .
Note
Because the TRL technique relies on the characteristic impedance of
transmission lines , the mathematically equivalent method LRM (for
line-reect-match) may be substituted for TRL. Since a well matched
termination is , in essence , an innitely long transmission line , it is well suited
for low (RF) frequency calibrations . A chieving a long line standard for low
frequencies is often times physically impossible .
Improving Raw Source Match and Load Match F or TRL*/LRM* Calibration
A technique that can be used to improve the raw test port mismatch is to add high quality
xed attenuators . The eective match of the system is improved because the xed attenuators
usually have a return loss that is better than that of the network analyzer . A dditionally , the
attenuators provide some isolation of reected signals . The attenuators also help to minimize
the dierence between the port source match and load match, making the error terms more
equivalent.
With the attenuators in place , the eective port match of the system is improved so that the
mismatch of the xture transition itself dominates the measurement errors after a calibration.
6-88 Application and Operation Concepts