Selectivity
Understanding Selectivity
Selectivity is quantified by the shape factor, defined as the ratio of bandwidth at a high attenuation level (e.g., -60 dB) to bandwidth at the -3 dB level. An ideal rectangular filter has a shape factor of 1:1, while practical filters range from 1.5:1 to 10:1 depending on design.
| 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 |
Higher selectivity requires more filter poles, higher-Q resonators, and precise tuning. Crystal, SAW, and cavity filters achieve excellent selectivity, while microstrip filters are limited by lower resonator Q values.
Frequently Asked Questions
What is shape factor?
Shape factor is the ratio of filter bandwidth at two different attenuation levels (e.g., BW at -60 dB vs BW at -3 dB). Values closer to 1.0 indicate better selectivity.
How is selectivity improved?
Adding more filter poles, using higher-Q resonator technologies, and using elliptic or quasi-elliptic topologies with transmission zeros improve selectivity.