When Should Port Deembedding Be Used on Wave Ports?

Problem/Description: ANSYS HFSS provides the ability to deembed port solutions by a specified distance. What does this deembedding do and when should it be used?

Solution:

With regard to wave ports, Port Deembedding in HFSS is a post-processing operation that changes the S-parameter’s reference plane based on the complex propagation constant calculated during the port’s solution. This allows the user to add or remove the effects of a straight section of transmission line from or to the S-Parameter values. Because the complex propagation constant is used, this operation will impact the S-Parameter’s magnitude and phase based on the transmission lines losses and propagation constant.

This operation should not be confused with removing the impact of different geometrical features from the S-Parameters such a bends, vias or tapers. Any discontinuity within the 3D geometry will impact the transmission line’s complex propagation constant which is not accounted for in the deembedding process. Only the propagation constant calculated at the port is considered.

Users should also be careful from confusing wave port deembedding with lumped port deembedding. Lumped port deembedding is used to remove parasitic effects from the lumped port’s solution. It does not move the S-Parameter’s reference plane.

Typical uses for deembedding include changing the S-parameter’s reference plane. This can be useful when evanescent modes are interacting with the port, but are not defined on the port. In such situations, wave ports will reflect the portions of the incident wave associated with these undefined modes back into the 3D solution creating the potential to affect the computed results. To minimize the impact of these undefined modes users can add a uniform section of transmission line effectively moving the wave port away from any discontinuity. Since discontinuities excite evanescent modes, the additional transmission line length allows these modes to decay before interacting with the port. Users can then deembed the wave port to move the S-parameter’s reference plane back to the port’s original position.

Since this phenomenon occurs in both Driven Modal and Driven Terminal solution types, this approach is equally valid for both cases.