Specifications

4.6 Practical compensation examples
The error sources present in a practical measurement setup are different for the configuration of
test fixtures, test cables, or circuits which may be connected between the instrument and the DUT.
Appropriate compensation methods need to be applied depending on the measurement configura-
tion used. Figure 4-13 shows examples of the compensation methods that should be used for typical
measurement setups.
4.6.1 Agilent test fixture (direct attachment type)
When an Agilent direct attachment type test fixture is used, open/short compensation is enough to
minimize the additional measurement errors. Since the characteristics of Agilent test fixtures can be
properly approximated by the circuit model shown in Figure 4-4, the open/short compensation
effectively removes the errors. Open/short/load compensation is not required as long as the funda-
mental measurement setup is made as shown in Figure 4-13 (a).
4.6.2 Agilent test cables and Agilent test fixture
When Agilent test cables and an Agilent test fixture are connected in series as shown in Figure 4-13 (b),
perform the cable length correction first. The cable length correction moves the calibration plane to
the tip of the test cables. Then, perform the open/short compensation at the DUT terminals of the
test fixture in order to minimize the test fixture induced errors.
4.6.3 Agilent test cables and user-fabricated test fixture (or scanner)
When Agilent test cables and a user-fabricated test fixture are connected in series as shown in
Figure 4-13 (c), perform the cable length correction first in order to move the calibration plane to
the tip of the test cables. The characteristics of the user-fabricated test fixture are usually unknown.
Thus, the open/short/load compensation should be performed to effectively reduce the errors even
if the test fixture has complicated residuals.
4.6.4 Non-Agilent test cable and user-fabricated test fixture
When a non-Agilent test cable and a user-fabricated test fixture is used, the 4TP measurement is
basically limited to the low frequency region. In the higher frequency region, this type of test configu-
ration may produce complicated measurement errors or, in the worst cases, cause the bridge unbal-
ance which disables measurements. When measurement setup is made as shown in Figure 4-13 (d),
the cable length correction cannot be used because it will not match the characteristics of the
non-Agilent cables. As a result, the calibration reference plane stays at the instrument’s UNKNOWN
terminals (as shown in Figure 4-1 (a).) Initially, verify that the bridge unbalance does not arise at
the desired test frequencies. Next, perform the open/short/load compensation at the DUT terminals
of the test fixture. This method can comprehensively reduce measurement errors due to the test
cables and fixture.
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