 |
Figure 1: This sequence diagram of the
classic SOLT 2-path calibration shows
the order of connections required. |
It was the best of S-parameter measurements, it was the worst of S-parameter measurements…and the difference was in the calibration. Calibrating a vector network analyzer (VNA) before making any measurements is required in order to reduce errors from imperfect channel matching, less than optimal directivity in the directional couplers and cable response issues. While VNAs are precisely calibrated at the factory, that calibration only extends to the front panel measurement ports. There will inevitably be drift on the internal paths over time. Also, any cables, adaptors or fixtures connected to the measurement ports must be characterized and de-embedded in order to make exact measurements of the device under test (DUT).
There are many possible calibration methods depending on the number of ports and paths being measured. For simplicity, let’s consider the common 2-port, 2-path calibration. This calibration method will yield a full set of S-parameters for the two ports: S11, S12, S21 and S22. It requires the use of a short, open, load and through (SOLT) calibration reference standard, along with the cables used in the test setup, as shown in Figure 1.
