Input Section  Coupling Matrix 

The coupling matrix may also
serve as an input device.
By selecting the "Plot
Using Coupling Matrix"
button, editing of couplings in the coupling matrix becomes possible.
The influence of the change
on the Sparameter characteristics can be inspected by clicking the
"Update" button. Changing couplings by: Using the 'slider':

Editing of coupling
coefficients is convenient for: 

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 xcoupling
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
(selected by mouse "rightclick" in the shown example above). If this coupling is zeroed,
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 xcoupling 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 are often found difficult to
obtain. Very often it turns out that the upper notch ends up lying 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. 
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