Technical data

5-28
Operating Concepts
Measurement Calibration
to mismatch and leakage in the test setup, isolation between the reference and test signal paths, and
system frequency response. The system cannot measure and correct for the non-repeatable random and
drift errors. These errors affect both reflection and transmission measurements. Random errors are
measurement variations due to noise and connector repeatability. Drift errors include frequency drift,
temperature drift, and other physical changes in the test setup between calibration and measurement.
The resulting measurement is the vector sum of the test device response plus all error terms. The precise
effect of each error term depends upon its magnitude and phase relationship to the actual test device
response.
In most high frequency measurements the systematic errors are the most significant source of
measurement uncertainty. Since each of these errors can be characterized, their effects can be effectively
removed to obtain a corrected value for the test device response. For the purpose of vector accuracy
enhancement, these uncertainties are quantified as directivity, source match, load match, isolation
(crosstalk), and frequency response (tracking). The description of each of these systematic errors follows.
Random and drift errors cannot be precisely quantified, so they must be treated as producing a cumulative
uncertainty in the measured data.
Directivity
Normally a device that can separate the reverse from the forward traveling waves (a directional bridge or
coupler) is used to detect the signal reflected from the test device. Ideally the coupler would completely
separate the incident and reflected signals, and only the reflected signal would appear at the coupled
output, as shown in Figure 5-20a.
Figure 5-20. Directivity
However, an actual coupler is not perfect, as shown in Figure 5-20b. 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 5-21. In a transmission measurement, the
source match error signal is caused by reflection from the test device that is re-reflected from the source.