Specifications
3.7 Test port extension in RF region
In RF measurements, connect the DUT closely to the test port to minimize additional measurement
errors. When there is an unavoidable need for extending the test port, such as in-circuit testing of
devices and on-wafer device measurement using a prober, make the length of test port extension as
short as possible. If the instrument has a detachable test head, it is better for accuracy to place the
test head near the DUT in order to minimize the test port extension length, and interconnect the test
head and the instrument using coaxial cables. (Observe the limit of maximum interconnection cable
length specified for the instrument.) Using a long test port extension will involve large residual
impedance and admittance of the extension cable in the measurement results, and significantly
deteriorate the accuracy even if calibration and compensation are completed.
Figure 3-19 shows an equivalent circuit model of the port extension. The inductance (Lo), resistance
(Ro), capacitance (Co), and conductance (Go) represent the equivalent circuit parameter values of
the extension cable. When the DUT’s impedance (Zx) is nearly 50 Ω, the test signal is mostly fed to
the DUT as the cable causes only a phase shift and (relatively small) propagation loss like a trans-
mission line terminated with its characteristic impedance. However, most likely the DUTs have a
different value from 50 Ω. If the impedance of the DUT is greater than that of Co, the test signal cur-
rent mainly bypasses through Co, flowing only a little through the DUT. Conversely, if the imped-
ance of the DUT is lower than that of Lo and Ro, the test signal voltage decreases by a voltage drop
across the cable and is applied only a little to the DUT. As a result, the cable residuals lead to mea-
surement inaccuracy and instability, particularly, in high-impedance and low-impedance measure-
ments. As illustrated in Figure 3-19, the Lo, Ro, Co, and Go not only get involved in the measure-
ment results (before compensation), but also affect measurement sensitivity. Note that the measur-
able impedance range becomes narrow due to port extension even though the calibration and com-
pensation have been performed appropriately.
Figure 3-19. Calibration plane extension
3-19