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Operating Concepts
Measurement Calibration
However, an actual coupler is not perfect, as shown in Figure 7-21b. A small amount of the
incident signal appears at the coupled output due to leakage as well as reflection from the
termination in the coupled arm. Also, reflections from the coupler output connector appear
at the coupled output, adding uncertainty to the signal reflected from the device. The
figure of merit for how well a coupler separates forward and reverse waves is directivity.
The greater the directivity of the device, the better the signal separation. System
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.)