What is a hairpin filter and when would I use it on a PCB?
Hairpin Filter Topology
The hairpin filter was developed to address the excessive length of edge-coupled half-wave filters by folding each resonator at its midpoint. The fold creates a U-shaped structure where the two arms are approximately λ/4 long each. The coupling between adjacent hairpin resonators occurs through the gaps between the open ends of adjacent U-shapes.
| Parameter | LC Lumped | Cavity | SAW/BAW |
|---|---|---|---|
| Q Factor | 50-200 | 1,000-20,000 | 500-2,000 |
| Frequency Range | DC-3 GHz | 0.1-40 GHz | 0.1-6 GHz |
| Insertion Loss | 1-6 dB | 0.2-2 dB | 1-4 dB |
| Size | Small (PCB) | Large (machined) | Very small (chip) |
| Tuning | Fixed or varactor | Mechanical screw | Fixed |
Frequently Asked Questions
How much smaller is a hairpin filter?
Approximately 50% shorter than an edge-coupled filter and 30-40% reduction in total area. The width increases slightly because the hairpin arms extend perpendicular to the filter axis. Overall, the hairpin is the best compromise between size and performance for PCB filters.
What bandwidth range works?
Hairpin filters are practical for 5-30% fractional bandwidth. Below 5%, the coupling gaps become too large (weak coupling) and the filter becomes dominated by the self-coupling of each hairpin. Above 30%, the coupling gaps become too small for reliable manufacturing.
Can I miniaturize further?
Yes. Use meandered resonators (folded multiple times), stepped impedance resonators (SIR) that use high-Z/low-Z sections to shorten the resonator, or load each resonator with a lumped capacitor. These techniques can reduce the size by another 30-50% at the cost of reduced Q and narrower bandwidth.