Agilent Technologies 8753ET User Manual page 378

Operating Concepts
Measurement Calibration
directivity is the vector sum of all leakage signals appearing at the analyzer receiver input.
The error contributed by directivity is independent of the characteristics of the test device
and it usually produces the major ambiguity in measurements of low reflection devices.
Source Match
Source match is defined as the vector sum of signals appearing at the analyzer receiver
input due to the impedance mismatch at the test device looking back into the source, as
well as to adapter and cable mismatches and losses. In a reflection measurement, the
source match error signal is caused by some of the reflected signal from the test device
being reflected from the source back toward the test device and re-reflected from the test
device. This is illustrated in Figure 7-22. In a transmission measurement, the source
match error signal is caused by reflection from the test device that is re-reflected from the
source. Source match is most often given in terms of return loss in dB: thus the larger the
number, the smaller the error.
Figure 7-22 Source Match
The error contributed by source match is dependent on the relationship between the actual
input impedance of the test device and the equivalent match of the source. It is a factor in
both transmission and reflection measurements. Source match is a particular problem in
measurements where there is a large impedance mismatch at the measurement plane.
(For example, reflection devices such as filters with stop bands.)
Load Match
Load match error results from an imperfect match at the output of the test device. It is
caused by impedance mismatches between the test device output port and port 2 of the
measurement system. Some of the transmitted signal is reflected from port 2 back to the
test device as illustrated in Figure 7-23. A portion of this wave may be re-reflected to
port 2, or part may be transmitted through the device in the reverse direction to appear at
port 1. If the test device has low insertion loss (for example a filter pass band), the signal
reflected from port 2 and re-reflected from the source causes a significant error because the
test device does not attenuate the signal significantly on each reflection. Load match is
usually given in terms of return loss in dB: thus the larger the number, the smaller the
error.
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