Technical data

5-29
Operating Concepts
Measurement Calibration
Source match is most often given in terms of return loss in dB: thus the larger the number, the smaller the
error.
Figure 5-21. 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 5-22. 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.
Figure 5-22. Load Match
The error contributed by load match is dependent on the relationship between the actual output
impedance of the test device and the effective match of the return port (port 2). It is a factor in all
transmission measurements and in reflection measurements of two-port devices. The interaction between
load match and source match is less significant when the test device insertion loss is greater than about 6
dB. However, source match and load match still interact with the input and output matches of the DUT,
which contributes to transmission measurement errors. (These errors are largest for devices with highly
reflective ports.)
Isolation (Crosstalk)
Leakage of energy between analyzer signal paths contributes to error in a transmission measurement,