Some filters tend to use resonators to replace LC circuits. It is said the reason is to get a high Q. Are resonators used only for narrow band filters?
Presume you ask about electroacoustic (piezo ceramic or crystal) resonators. LC, transmission line or cavity filters are resonators as well.
The high Q and low tolerance resonators can be utilized to also model a precise transmission in a high order filter, like the FM IF filters mentioned by Audioguru. SAW filters are an alternative way to achieve this goal.
Only narrow band filters? No. A properly designed resonator filter can give a BW of 15 - 20%, just like "regular" filters. Most microwave filters only use resonators. Resonators are used for the high Q, which will lead to lower insertion loss, since there is less loss in the resonator. Also, at higher freqs, lumped components cannot be realized (try to find a 5fH inductor), not to mention the parasitics.
Only narrow band filters? No. A properly designed resonator filter can give a BW of 15 - 20%, just like "regular" filters. Most microwave filters only use resonators. Resonators are used for the high Q, which will lead to lower insertion loss, since there is less loss in the resonator. Also, at higher freqs, lumped components cannot be realized (try to find a 5fH inductor), not to mention the parasitics.