Output CPM

Input Section - Coupling Matrix

The coupling matrix may also serve as an input device.

By selecting the "From Coupling Matrix" button, editing of couplings in the coupling matrix becomes possible.

An entry in the coupling matrix may be edited by selecting it by a click with the mouse. The present value may then be deleted by use of the keyboard "Backspace" button. Once deleted a new value can be entered.

Important: The entered new value is first accepted when the "Enter" or "Return" button is hit.

The influence of the change on the S-parameter characteristics may be inspected by clicking the "Update" button.

Editing of coupling coefficients may be handy for:

zeroing of very weak couplings in order to see if they can be omitted in a physical filter
change of coupling signs
investigation of other N+2 coupling matrices e.g. in the literature
analysis of where unintended couplings may arise from in a physical filter
sensitivity analysis of critical couplings

Example:

A filter with two symmetrically positioned notches is designed. The synthesized characteristic is shown below:

It is well known that such a characteristic can be implemented by a single negative x-coupling bypassing two resonators. In the present case this coupling is found between resonator 2 and 5 in the coupling matrix above.

It is, however, noted that the synthesizer has actually added an extra coupling between resonator 3 and 5 (marked with red circle).
This coupling is more than 20 times weaker than the other x-coupling.

If we manually set this coupling to zero, the characteristic below is obtained:

It is seen that zeroing of the weak coupling has almost no influence on the filter characteristic, and this coupling can therefore be omitted if the filter is going to be manufactured.

The characteristic can therefore be implemented by use of a single negative x-coupling which bypasses two resonators.

The reason that the synthesizer has found it necessary to include this extra small coupling originates from limited numerical accuracy in the calculations and algorithms, which lie behind the synthesis.
Zeroing of the mentioned coupling could also be achieved by adjustment of one of the notch frequencies by a very small amount, thereby moving the characteristic slightly away from perfect symmetry.

In aperture coupled combline filters the above symmetrical characteristic is often found difficult to obtain. Very often it turns out that the upper notch ends up lying much closer to the pass band than the lower notch does.
It can be verified by insertion of weak positive couplings that this asymmetry may originate from unintended coupling between resonators 3 and 5 and/or between 2 and 4.