Coupling matrices model electro-magnetic couplings between resonators. These couplings are modeled as ideal immitance-inverters that has a phase shift of 90 degrees and a fixed impedance. In reality this is only true at the center frequency since practical immitance-inverters can only achieve a 90 degree phase shift of a certain bandwidth. A synthesized response in Couplings Designer is therefore accurate with filters of bandwidth 10% or less. Wide-band filters can still be synthesized using coupling matrices but will most likely require a lot of tuning in the realization process.
Due to the advanced math involved in synthesizing a coupling matrix the numerical algorithms used will break down when the filter order is about 12 or higher. It is still possible to seed higher order filters but the response will be more or less distorted depending on your other settings.
Source-load coupling in the topology editor
The topology transformer does not support source-load coupling. This means that source-load coupling can not be eliminated and absorbed in other couplings. If however source-load coupling is present the residues shown while tranforming will be the value of this normalized coupling. The tranformation process will still converge as usual and leave the source-load coupling untouched.
Back to the Couplings Designer documentation.