8.3     S-Parameters

The  button in Simulation tab and Postprocessings->S-Parameters... command from main menu invoke S-Parameters dialogue with settings for S-Parameters post-processing.

When choosing the option of S-Parameters calculation we must decide upon the range of frequencies we are interested in, and the frequency step. The number of frequency points whereat the S-Parameters are calculated increases to some extent the computing time and memory required. However, the computing time and memory used for S-Parameters calculations at several hundreds of the frequency points usually does not exceed a few percent of the total computing time and memory needed to run the electromagnetic simulation. Thus the choice of this number in the range between one hundred and five hundred seems reasonable for most applications.

In the S Parameters extraction frame we have the choice of several options:

·       SK1 at reference planes. In this case we excite the circuit only from port number 1 and calculate the incident (a_k) and reflected (b_k) waves at each port. This permits to calculate S_k1 = b_k/a₁. Note that:

o        The simple extraction produces results contaminated by numerical reflections from imperfect absorbing boundaries.

o        If the circuit is declared as reciprocal N-port, it is possible to correct the value of S₁₁, applying the expanded formula S₁₁=b₁/a₁-S₁₂a₂/a₁-S₁₃a₃/a₁-.... The software enforces S_k1=S_1k and calculates S₁₁=b₁/a₁-b₂a₂/a₁²-b₃a₃/a₁²-....

o        If the circuit is declared as reciprocal lossless 2-port, it is also possible to correct the value of S₂₁, applying the formula S₂₁=b₂/a₁-S₂₂a₂/a₁. The software enforces |S₂₂|=|S₁₁| and arg(S₁₁)+arg(S₂₂)-2arg(S₁₂)= ±p.

o        The software will not verify your declaration regarding reciprocity and losses, but it will ignore the declaration of lossless 2-port if the number of ports if different.

·       Smn at reference planes. This is a general case in which we perform N simulations exciting the structure from N different ports, and then compose the complete S-matrix from N sets of partial results. Each of these sets corresponds to excitation from port i when the software calculates the parameters S_ki in the same way as the parameters S_k1 are calculated in the Sk1 at reference planes postprocessing. When all N sets are available, the software corrects all the S-matrix elements for imperfect absorbing at all ports (reciprocity is irrelevant at this stage). Thus the final results of S_mn calculation may somewhat differ from the N sets of partial results. There are two regimes of running the Smn at reference planes postprocessing:

o        In the Sequential regime the software performs N consecutive simulations (with excitation from each of the N ports). Each simulation lasts for a predefined number of FDTD iterations, specified by the user in the Iterations per port box. Note that during one simulation we obtain S_k1 elements like in the Sk1 at reference planes postprocessing, and these parameters (either uncorrected or partially corrected for imperfect absorbing boundaries) are being displayed. Only after completing the N simulations with excitation from N consecutive ports the complete S-matrix will be assembled, correcting all the S-matrix elements for imperfect absorbing at all port. Thus the final results of S_mn calculation may be somewhat different from the intermediate results displayed during calculation.

o        In the Multisimulator regime, the software runs N instances of internal simulator objects in the QW-Simulator application, with a different exciting port in each of them. An apparent disadvantage of this regime resides in the increased memory requirements. However, its important advantage is the possibility of the on-line monitoring of the full corrected S-matrix, calculated for the current number of FDTD iterations.

·       Gamma K at ref. planes. It allows computing reflection coefficients G at several reference planes simultaneously during a single simulation run. It can be applicable when a multi-source network is considered with all the ports operating as sources simultaneously and, consequently, the scattering matrix cannot be computed.

correction for TEM transmission lines option allows switching on or off correction for S-Parameters extraction close to DC.

Attention:

Only in the circuit, which is not declared to be reciprocal, the reflection coefficient is calculated independently of the outputs. In other cases it is corrected for the reflection on the output ports. In those cases any errors of analysis at the output (wrong port definition, wrong template and so on) may strongly influence also the S₁₁ calculation.

Detailed discussion regarding S-Parameters post-processing is given in S-Parameters.