As discussed in Noise Figure (Option 41), handling of the DUT gain is an important part of noise figure computations and, in the case of 3- and 4-port DUTs, the S-parameters will come in the form of an .s3p or .s4p file. If the DUT outputs are to be treated as single-ended, then the analysis process may just be a simple extraction of the 2-port paths of interest. If it is to be treated as a differential or common-mode measurement, then the gain must be calculated in those terms. To further complicate the practicalities, the port assignment in the .s3p/.s4p file may not match how the DUT is connected to the noise measurement ports.
Port Assignments
The DUT S-parameter file loader in noise figure ALWAYS uses the following port assignment for .s4p: DUT inputs are ports 1 and 2 connected, respectively, to output ports 3 and 4. When loading the file, a port swap tool is available to let the user align the port configuration used in the actual file to the port assignment (shown below) used by the noise figure application.
Port Assignments When Loading .s4p Files for DUT S-parameters
If the data was saved using a different port configuration, there is a port reassignment tool available.
For the .s3p case with an output pair, the following assignment is ALWAYS used: ports 2 and 3 form the output.
Port Assignments When Loading .s3p Files for DUT S-parameters.
If the data was saved using a different port configuration, there is a port reassignment tool available.
As with Option 041 Noise Figure, the loaded DUT S-parameters can be plotted on-screen (as static memory data) but now the 3- and 4-port S-parameters can be plotted if loaded (even if the VNA in use is a 2-port instrument). The variables are available under the RESPONSE menu.
Gain Definitions
For differential insertion gain, the mixed mode parameter Sd2d1 is used. The mode conversion parameter Sd2c1 is also needed and the net insertion gain is |Sd2d1|2 + |Sd2c1|2. Similarly for common-mode, the net insertion gain is |Sc2c1|2 + |Sc2d1|2. The concept on these gain configurations relies on the use of uncorrelated terminations at temperature T0. This will be discussed further in a later section but, from a gain perspective, it means that equal noise inputs at the common and differential modes will exist. Thus, the gain of interest has to take into account the output power in a given mode due to both differential and common-mode noise inputs. The mixed mode parameters are discussed in more detail in Multiport Measurements, but, for the port assignments listed above, they are defined here as
Equation 20‑1.
For the 3-port case with a mixed-mode DUT output, there are no mode conversion issues and the insertion gains are |Sd1|2 and |Sc1|2 for differential and common-mode respectively. The mixed-mode definitions of the underlying parameters are
Equation 20‑2.
The use of available gain is more correct for noise figure analysis, as has been discussed previously and the multiport equivalent of this added level of vector correction is also available. These definitions are in Eq. 20‑3 and, as in the 2-port case, account for the power delivery impact of DUT output mismatch.
Equation 20‑3.
For the SE-in, diff out 3-port DUT case, things simplify:
Equation 20‑4.
The underlying definitions for the reflection mixed mode parameters used here are (again DUT output ports are 3 and 4 for the 4-port case and 2 and 3 for the 3-port case)
Equation 20‑5.
For 2-port DUT analysis, the available gain used is, of course |S21|2/ (1-|S22|2) where 1->2 is the path of analysis.
