What is the difference between sensitivity and selectivity in a receiver?
Sensitivity and Selectivity Compared
Sensitivity and selectivity are the two most fundamental specifications of any receiver. Sensitivity determines whether a weak desired signal can be detected at all. Selectivity determines whether a desired signal can be received in the presence of nearby interferers.
Sensitivity depends on the receiver noise figure and the post-detection bandwidth. Improving the noise figure or narrowing the bandwidth lowers the noise floor and improves sensitivity. However, the bandwidth cannot be narrower than the signal bandwidth without causing distortion. This sets a fundamental limit: the best achievable sensitivity for a given signal format is determined by the signal bandwidth and the receiver noise figure.
Selectivity depends on the filter characteristics in the receiver chain. High-selectivity receivers use high-order filters with steep roll-off (high shape factor) to attenuate nearby signals while passing the desired signal with minimal distortion. Crystal, SAW, and ceramic filters provide different performance levels, and digital filters offer nearly ideal selectivity characteristics.
Frequently Asked Questions
Can I improve sensitivity without affecting selectivity?
Yes. Reducing the noise figure (better LNA) improves sensitivity without changing selectivity. The filter characteristics remain the same; only the noise floor changes.
Can I improve selectivity without affecting sensitivity?
Partially. A better filter (steeper skirts, same passband bandwidth) improves selectivity. However, any filter adds insertion loss, which degrades sensitivity unless compensated by additional gain.
Which matters more in dense environments?
Selectivity often matters more because strong adjacent-channel signals can desensitize the receiver through intermodulation and blocking. In these environments, the practical sensitivity is limited by interference, not thermal noise.