7-71
Operating Concepts
TRL*/LRM* Calibration (ES Models Only)
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 fixed
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.
Figure 7-45 Typical Measurement Set up
If the device measurement requires bias, it will be necessary to add external bias tees
between the fixed attenuators and the fixture. The internal bias tees of the analyzer will
not pass the bias properly through the external fixed attenuators. Be sure to calibrate with
the external bias tees in place (no bias applied during calibration) to remove their effect
from the measurement.
Because the bias tees must be placed after the attenuators, they es sentially become part of
the fixture. Their mismatch effects are the same for source match and load match, so the
TRL CAL routine will correct for their effects. Although the fixed attenuators improve the
raw mismatch of the network analyzer system, they also degrade the overall measurement
dynamic range.
This effective mismatch of the system after calibration has the biggest effect on reflection
measurements of highly reflective devices. Likewise, for well matched devices, the effects
of mismatch are negligible. This can be shown by the following approximation:
Reflection magnitude uncertainty = ED + ERS11 + ES(S11)2 + ELS21S12