Читать книгу Handbook of Microwave Component Measurements - Joel P. Dunsmore - Страница 97

In some VNAs the source is switched between ports using a test set switch, which can come before or after the reference channel splitter. The termination of this switch provides the load match of the port when the source is not active on that port. This load match is not the same as the source‐match (ratio or power) and so some advanced calibration techniques that rely on the port match being consistent whether the port is a source or a load must be modified, as discussed in the next chapter. If the switch comes before the reference channel splitter, there will be a reference channel receiver for each port (four‐receiver VNA, Figure 2.2 lower). If the switch comes after the reference channel splitter (three‐receiver VNA, Figure 2.2 upper), the reference channel is shared between ports. It samples the source signal only when the source is active. This three‐receiver architecture does not support some calibration methods, such as thru‐reflect‐line (TRL) and so modifications and compromises to the calibration methods must be made.

The short explanation for the difficulty is that TRL calibration methods require measuring the load match of port 2, when port 1 is active. To do this measurement, the ratio of a2/b2 is acquired during the thru step. But there is no a2 receiver available in the three‐receiver architecture. Modifications can be made that assume the source‐match and load match of the port are identical, but this case is not common unless attenuation is added after the reference channel split. Attenuation added reduces the difference between source and load match at a port by twice the attenuation value. Pre‐characterization of the difference in source and load match, called the delta‐match, can be performed and removes the need for characterization at the time of calibration. This allows three‐receiver architectures to support the same calibration as four‐receiver architectures and is found in some of the more modern low‐cost analyzers.