# Quarter-Wave Bandstop Filter Design - simple

This tutorial tells you how to design a simple quarter-wave bandstop stub
filter with simpleEMS. This filter stops the frequencies in one band and passes
the others.

```{note}
You must have simpleEMS and openEMS on your computer. See
[Installation](installation.md).
```

Make a file with the name `quarterwave_bandstop_filter.py`.

## Import the modules

Import `QuarterWaveFilterParams`, `BandStopQuarterWaveFilter`, and
`setup_simulation` from `simpleEMS`.

```{literalinclude} ../../../examples/BandStopQuarterWaveFilter.py
:language: python
:start-after: IMPORTS
:end-before: IMPORTS
```

`QuarterWaveFilterParams` holds the parameters of the filter and of the
simulation.

```{seealso}
- {class}`simpleEMS.quarterwave_stub_filter.QuarterWaveFilterParams`
```

`BandStopQuarterWaveFilter` makes the filter.

```{seealso}
- {class}`simpleEMS.quarterwave_stub_filter.BandStopQuarterWaveFilter`
```

`setup_simulation` makes the CSXCAD geometry and the FDTD solver. It also sets
the frequency range of the simulation.

```{seealso}
- {func}`simpleEMS.sim_tools.setup_simulation`
```

## Set the parameters

```{literalinclude} ../../../examples/BandStopQuarterWaveFilter.py
:language: python
:start-after: PARAMS
:end-before: PARAMS
```

`QuarterWaveFilterParams` holds the centre frequency, the bandwidth, the type
of response, the order of the filter, and the properties of the substrate.

## Set up the simulation

```{literalinclude} ../../../examples/BandStopQuarterWaveFilter.py
:language: python
:start-after: SETUP
:end-before: SETUP
```

`setup_simulation` prepares the FDTD solver and the CSXCAD geometry. It returns
a `SimSetup` with the `CSX`, `FDTD`, and `freqs` attributes.

## Build the filter

```{literalinclude} ../../../examples/BandStopQuarterWaveFilter.py
:language: python
:start-after: BUILD
:end-before: BUILD
```

These commands build the substrate, the ground plane, the series lines, the
stubs, the ports, and the mesh. Then they show the structure in AppCSXCAD.
Close the AppCSXCAD window to continue. The filter is now ready for the
simulation.

## Run the simulation

```{literalinclude} ../../../examples/BandStopQuarterWaveFilter.py
:language: python
:start-after: SIMULATE
:end-before: SIMULATE
```

This command runs the openEMS solver. Wait until the solver stops.

## Show the results

```{literalinclude} ../../../examples/BandStopQuarterWaveFilter.py
:language: python
:start-after: PPROCESS
:end-before: PPROCESS
```

These commands show the S11 and the S21 curves. Use them to examine the
stopband of the filter.

## Export the results

You can write the model to the Gerber format. Send these files to a PCB
manufacturer.

```{literalinclude} ../../../examples/BandStopQuarterWaveFilter.py
:language: python
:start-after: EXPORT
:end-before: EXPORT
```

## Complete script

The complete script is below. It designs the filter, runs the simulation, and
shows the results.

```{literalinclude} ../../../examples/BandStopQuarterWaveFilter.py
:language: python
```