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question about filter and resonator

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danda821

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I am wondering if it is possible to design a filter using low Q resonator (say 20) to achieve good IL (say 3dB). I saw some paper using low Q L (8,eight) and C build filter with IL of 2.5dB. But I saw some textbook claims if the Q is bad the filter using coupled resonators will have big IL. I don't know if that is true for all filters, and what is the relationship between Q and IL.
 

rautio

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For wide bandwidth you want a low loaded Q. This is the Q with input and output terminated in the design resistance, often 50 Ohms. To get the low loaded Q, you tightly couple into the resonator.

To get low insertion loss, you want a high unloaded Q. Unloaded Q is the Q of the resonator with no external coupling (in practice, very very light external coupling). The unloaded Q is determined by metal and dielectric loss around the resonator.

So for low insertion loss and wide bandwith, you want a high (unloaded) Q resonator to which you tightly couple input and output. The tight coupling give you low loaded Q and wide bandwidth.
 

danda821

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So is it possible to achieve 3dB IL using resonators with unloaded Q of about 20?
 

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