How do I design an open loop ring resonator bandpass filter on a PCB?
Ring Resonator Filter Design
The open-loop ring resonator filter offers significant size reduction compared to conventional half-wave or quarter-wave resonator filters because the loop structure efficiently uses PCB area. A square ring resonator with perimeter = λg has approximately half the linear extent of a half-wave resonator and better radiation properties because the opposing currents in the loop partially cancel far-field radiation.
| 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 |
- Performance verification: confirm specifications against the application requirements before finalizing the design
- Environmental factors: temperature range, humidity, and vibration affect long-term reliability and parameter drift
- Cost vs. performance: evaluate whether the application demands premium components or standard commercial grades
- Interface compatibility: verify impedance, connector type, and mechanical form factor match the system architecture
- Margin allocation: include sufficient design margin to account for manufacturing tolerances and aging effects
Frequently Asked Questions
How much size reduction?
A dual-mode ring resonator provides 2-pole filtering in the area of a single resonator, approximately 50% size reduction compared to two separate hairpin resonators. For a 4-pole filter using two dual-mode rings, the size reduction is 40-60% compared to a conventional 4-element hairpin filter.
What bandwidth is achievable?
Ring resonator filters work for 2-15% fractional bandwidth. The dual-mode coupling (perturbation size) controls the bandwidth. Very narrow bandwidth (< 2%) requires very small perturbations that are sensitive to manufacturing tolerance. Wide bandwidth (> 15%) requires large perturbations that distort the ring symmetry.
Can I create transmission zeros?
Yes. The dual-mode perturbation naturally creates transmission zeros near the passband. The zero locations are controlled by the perturbation type and position. A notch perturbation at a corner places zeros close to the passband (steep skirts), while a stub perturbation provides more control over zero placement.