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Using MFilter for designs with round rod slabline t-lines

Question asked by JERRY_MARKETOS on Jun 8, 2004
Occasionally, we receive questions about how to use the MFilter synthesis
module for design of filter using slabline type transmission lines.

Firstly, let us clarify what is meant by slabline;  This means round rods for the resonators,
usually in air Er=1 dielectric (but not mandatory).  This filter is most often realized by machined
metal housing or cavity with round metal resonators that resemble tubes or metal rods.

We sometimes receive a question when a user desires to synthesize such a design regarding
the entry variables given to the synthesis module (or, perhaps more importantly, trying to determine
what are the resulting dimensions of the synthesized filter).   Let's walk through a typical slabline design.

For round rod slabline design, the resulting filter is not planar in vertical axis dimension,
so the concept of making a planar layout becomes invalid.  For this reason, Genesys will
warn us and thus the "create layout" option on the "options" tab should be unchecked.

Usually the dielectric used with a round rod slabline filter is the air in the housing / cavity,
so Er will be 1.  (This is not mandatory - you can use other Er with Genesys if the cavity
will be filled with something other than air).

We must also define the height of the cavity.  Height is a user specified number, and represents
the total height from upper ground plane to lower ground plane of the cavity.  In the synthesized
filter, the cross section of rod diameter exists evenly spaced within the cavity height.  Said differently,
half the cavity height exists above and below the center point of the rod diameter.

This total cavity height and dielectric Er are specified in a substrate.
Add a substrate to the Genesys substrate folder (right-click the substrate folder and choose "add substrate"),
and specify the height and Er. in the substrate. (specify the other relevant parameters, also).

Next, run the MFilter synthesis module and provide the desired inputs on
the topology and settings tabs.  Topology is often combline.  On the "options"
tab, remember to uncheck "create layout", and click the button
for "select manufacturing process" and choose round rod slabline.

Mfilter will synthesize the filter.  Be sure to inspect the warnings in case the resulting
synthesis falls outside the regions of acceptable inputs for the models.  (In such a case,
you can often adjust the entry parameters to return the design within the model parameter boundaries)

Once the filter synthesis is complete (and you have done optimization, etc.), there are a couple
of ways to view the resulting dimensions:

Method_1) First, doubleclick the components on the schematic to view their properties.  There,
you will see either direct numeric entry that represents a dimension you desire, or you will see
a variable name.  If a variable name, note what the names are, then close the component
dialogue box and open the equations window (in Genesys folder tree) to view the value
of that equation.

Method_2) Another way to see the variable values is to right-click the outputs folder in the
Genesys file tree and choose "add variable output".  This will add a window that shows
(in real time) the value of the variables.  Don't forget, if some of the parts happen to have
direct numeric entry from the synthesis (and they often do) you will need to view the part
dialogue (as mentioned in method1) to view those direct numeric entries.