Coupled Line Filter
Understanding Coupled-Line Filters
Coupled-line filters are the default bandpass filter technology for PCB-integrated circuits. They are easy to design, simulate, and fabricate using standard PCB processes. Their performance is limited by the PCB substrate loss (Q) but is adequate for many applications.
| Filter Type | Q Factor | Frequency Range | Size |
|---|---|---|---|
| LC Lumped | 50-200 | DC-3 GHz | Small (PCB) |
| Cavity | 1,000-20,000 | 0.1-40 GHz | Large |
| SAW | 500-2,000 | 0.1-3 GHz | Very small |
| BAW/FBAR | 1,000-3,000 | 0.5-6 GHz | Chip-scale |
Coupled-Line Filter Types
- Edge-coupled: Resonators side by side, coupled through fringing fields at the edges. Most common microstrip implementation.
- Broadside-coupled: Resonators on different layers, coupled through the substrate. Tighter coupling for wider bandwidth.
- End-coupled: Half-wave resonators coupled through gaps at their ends. Simple but weak coupling (narrowband).
- Hairpin: Folded half-wave resonators for compact size. Standard for commercial implementations.
Frequently Asked Questions
What is a coupled-line filter?
A coupled-line filter uses parallel microstrip or stripline resonators coupled through electromagnetic fields to create bandpass filtering. The coupling gap sets bandwidth; resonator length sets center frequency. The most practical planar filter topology.
What determines coupled-line filter performance?
Filter Q (and thus insertion loss) is limited by the substrate loss tangent and conductor loss. On Rogers RO4003, typical Q is 100-200. On alumina, Q reaches 300-500. Higher Q means lower insertion loss and sharper rolloff.
What is a hairpin filter?
A hairpin filter folds the half-wave coupled-line resonators into U-shapes, reducing the filter length by nearly half. This is the most common implementation for practical PCB filters. The fold does not significantly affect RF performance.