matlab-design-pcb-filter

# Designing RF Filters

## When to Use

- Designing bandpass filters (coupled-line, hairpin, open-loop, combline, interdigital, SIW)
- Designing lowpass filters (stepped-impedance)
- Designing bandstop or notch filters (spurline, stub-based)
- Extracting coupling matrices from measured S-parameter data (measuredFilter)
- Selecting a filter topology for a given bandwidth, selectivity, or size requirement

## When NOT to Use

- Designing transmission lines for impedance control — use `matlab-design-pcb-txline`
- Designing couplers or splitters — use `matlab-design-pcb-coupler`
- Designing passive components (inductors, capacitors, baluns) — use `matlab-design-pcb-passive`
- Setting up substrate or conductor materials — use `matlab-manage-pcb-material`
- Optimizing filter dimensions after design — use `matlab-optimize-pcb-design`

## Typical Workflow

1. **Before:** `matlab-manage-pcb-material` — set up substrate and conductor
2. **This skill:** Design the filter (catalog object or custom geometry)
3. **Check mesh/memory:** `memoryEstimate(obj, fc, 'RetainMesh', true)` — inspect auto-mesh density before committing to a full solve
4. **After:** `matlab-analyze-em` — validate S-parameters → `matlab-optimize-pcb-design` — tune dimensions → `matlab-write-pcb-layout` — export Gerber

## Quick Reference — Filter Selection

| Filter Object | Type | Poles | Best For |
|---------------|------|-------|----------|
| `filterCoupledLine` | Bandpass | 2–8 | General microstrip BPF |
| `filterHairpin` | Bandpass | 2–8 | Compact BPF, folded resonators |
| `filterOpenLoop` | Bandpass | 4/6/8 | Compact quasi-elliptic |
| `filterCombline` | Bandpass | 2–6 | Narrow-band, high-Q |
| `filterInterdigital` | Bandpass | 2–8 | Wideband, good stopband |
| `filterStepImpedanceLowPass` | Lowpass | 3–9 | Distributed LPF |
| `filterStub` | LP/HP/BS | N stubs | Flexible stub topology |
| `filterSpurline` | Bandstop | 1–2 | Compact notch filter |
| `SIWFilter` | Bandpass | 2–6 | High-Q waveguide-in-PCB |
| `measuredFilter` | Bandpass | N | Model extraction from measurements |

## Bandpass Filters

### Coupled-Line Filter

```matlab
f = filterCoupledLine;
f = design(filterCoupledLine, 3e9);    % Design at 3 GHz
show(f);
sp = sparameters(f, linspace(1e9, 5e9, 101), 'SweepOption', 'interp');
rfplot(sp);
```

Key properties: `FilterOrder`, `CoupledLineLength`, `CoupledLineWidth`, `CoupledLineSpacing`, `PortLineLength`, `PortLineWidth`.

### Hairpin Filter

Folded coupled-line resonators for compact size:

```matlab
f = design(filterHairpin, 3e9);
show(f);
memoryEstimate(f, 3e9, 'RetainMesh', true);  % Check mesh density before solving
sp = sparameters(f, linspace(1e9, 5e9, 101), 'SweepOption', 'interp');
rfplot(sp);
```

Key properties: `FilterOrder`, `CoupledLineLength`, `CoupledLineWidth`, `CoupledLineSpacing`, `PortLineLength`, `PortLineWidth`, `Spacing`, `ResonatorOffset`, `FeedOffset`.

### Chebyshev Response

Pass `FilterType` and `RippleFactor` to `design()` for equiripple passband response:

```matlab
f = design(filterHairpin, 1.8e9, FBW=10, FilterType='Chebyshev', RippleFactor=0.5);
```

`FilterType` options: `'Butterworth'` (default), `'Chebyshev'`. `FBW` sets fractional bandwidth (%). `RippleFactor` sets passband ripple in dB (default 0.5) — only applies to Chebyshev.

### Fifth-Order Hairpin

```matlab
f = filterHairpin;
f.FilterOrder = 5;
f = design(f, 2.4e9);
show(f);
```

### Open-Loop Filter

Quasi-elliptic response with cross-coupling:

```matlab
f = filterOpenLoop;
f.NumPoles = 6;
f.FeedOffset = 0.5e-3;
show(f);
sp = sparameters(f, linspace(1e9, 5e9, 101), 'SweepOption', 'interp');
rfplot(sp);
```

**Key Properties:** `NumPoles` (4/6/8), `ResonatorLength`, `ResonatorWidth`, `SplitGap`, `GapHorizontal`, `GapVertical`, `FeedOffset`, `CoupledResonatorGap`, `QuadrupletGap`, `QuadrupletOffset`.

### Combline Filter

Short-circuited resonators, excellent for narrow-band. **Note:** `filterCombline` does not have a `design` function — set properties manually:

```matlab
f = filterCombline;
f.FilterOrder = 3;
f.Height = 1.6e-3;
show(f);
```

**Key Properties:** `FilterOrder`, `ResonatorLength` (scalar or vector), `ResonatorWidth`, `ResonatorSpacing` (scalar or vector), `ResonatorOffset`, `FeedOffset`, `Capacitor` (loading capacitance — distinctive to combline).

### Interdigital Filter

Alternating short-circuited resonators, wideband. **Note:** `filterInterdigital` does not have a `design` function — set properties manually:

```matlab
f = filterInterdigital;
f.FilterOrder = 4;
f.Height = 1.6e-3;
show(f);
sp = sparameters(f, linspace(3e9, 7e9, 101), 'SweepOption', 'interp');
rfplot(sp);
```

**Key Properties:** `FilterOrder`, `ResonatorLength` (scalar or vector), `ResonatorWidth` (scalar or vector), `ResonatorSpacing` (scalar or vector), `ResonatorOffset`, `ViaDiameter` (scalar or vector — distinctive to interdigital), `FeedOffset`, `IsShielded`, `Connector`.

## Lowpass Filters

### Stepped-Impedance Lowpass

```matlab
f = filterStepImpedanceLowPass;
f = design(filterStepImpedanceLowPass, 2.5e9);
show(f);
sp = sparameters(f, linspace(0.1e9, 5e9, 101), 'SweepOption', 'interp');
rfplot(sp);
```

Key properties: `FilterOrder`, `HighZLineWidth`, `LowZLineWidth`, `HighZLineLength`, `LowZLineLength`.

## Bandstop / Notch Filters

### Spurline Filter

Compact notch using coupled-line section on one side:

```matlab
f = filterSpurline;
show(f);
sp = sparameters(f, linspace(1e9, 6e9, 51), 'SweepOption', 'interp');
rfplot(sp);
```

Double spurline for deeper rejection:

```matlab
f = filterSpurline;
f.LineType = 'Double';
show(f);
```

**Key Properties:** `LineType` (`'Single'`/`'Double'`), `CoupledLineLength`, `CoupledLineWidth`, `CoupledLineSpacing`, `LineGap` (gap between coupled line and output line — distinctive to spurline), `IsShielded`, `Connector`.

### Stub Filters (Open/Short)

The `filterStub` object supports open-circuit stubs (bandstop) and short-circuit stubs (highpass):

```matlab
f = filterStub;
f.StubLength