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.
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.