Microwave Cavity
Understanding Microwave Cavities
Microwave cavities are the highest-Q resonators available at microwave frequencies (excluding superconducting structures). Their combination of high Q, stability, and power handling makes them indispensable for base station filters and precision oscillators.
| 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 |
Cavity Resonator Properties
- Q factor: 5,000-10,000 (aluminum), 10,000-30,000 (silver-plated), 50,000+ (temperature-controlled).
- Resonant modes: TE or TM modes. The dominant mode depends on cavity shape (cylindrical: TM010, rectangular: TE101).
- Tuning: Adjustable screw or plunger changes the effective cavity volume, tuning the resonant frequency.
Cavity Filter Applications
- Cellular base station: Cavity duplexers separate TX and RX bands. Required for high-power multichannel sites.
- Satellite: Input multiplexers (IMUX) and output multiplexers (OMUX) in satellite transponders.
- Instruments: Wavemeters (cavity frequency meters) for quick frequency identification.
Frequently Asked Questions
What is a microwave cavity?
A microwave cavity is a metallic enclosure resonating at frequencies determined by its dimensions. Q factors of 5,000-50,000+ make cavities the highest-Q microwave resonators. Used for filters, oscillators, and frequency meters.
How does cavity Q compare to other resonators?
Lumped LC: Q 50-200. Microstrip: Q 100-300. Dielectric resonator: Q 5,000-20,000. Metal cavity: Q 5,000-50,000. Superconducting cavity: Q > 10^9. Higher Q = sharper resonance, lower loss, better filter selectivity.
How do you tune a cavity?
Insert a metallic screw or plunger into the cavity. Screw into E-field region: capacitive loading, frequency decreases. Screw into H-field region: inductive loading, frequency increases. Typical tuning range: 1-5% of center frequency.