HP 8753E manual Source match and load match, Frequencies is often times physically impossible

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 (ELF). 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