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Loaded and unloaded Q's values in filter design

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ashishchandra

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well i read it in other topic in this forum that filter having very narrow bandwidth cant be designed by microstrip technology because of very high unloaded Q required in designing those filter...i try to find the reason for it but i couldn't...

So, can anyone tell me why it is so???and what are the limits on Q in micristrip technology.???and also if i want to design a filter having very narrow bandwidth say at 2Ghz having a band of 20Mhz what design methodologies are available and which is best if i want to make it tunable also......

waiting for responses..........
 

ashishchandra

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ashishchandra said:
well i read it in other topic in this forum that filter having very narrow bandwidth cant be designed by microstrip technology because of very high unloaded Q required in designing those filter...i try to find the reason for it but i couldn't...

So, can anyone tell me why it is so???and what are the limits on Q in micristrip technology.???and also if i want to design a filter having very narrow bandwidth say at 2Ghz having a band of 20Mhz what design methodologies are available and which is best if i want to make it tunable also......

waiting for responses..........
 

vfone

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You are looking for a 1% bandwidth of central frequency.
This is almost impossible to do using microstrip components. Microstrip filters generally have 10% up to 25% bandwidth of central frequency.
The only way to realize this filter is to use cavity topology. This can be tunable, but having big dimensions.
 

Albertoooo

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You can design filters with 1% FBW using ustrip, but you will have very high insertion lossess and rounding in the passband, due to the limited Q of the resonators.

The unloaded Q of a resonator is related with the lossess, in the case of ustrip the losses of the conductor (gold, copper...) and the dielectric (alumina, whatever...). The width of the ustrip resonator is also related with its unloaded Q (more width, less lossess, higher Q).

To design low FBW filters you can use for example hairpin. There are a lot of books about rf filters (Pozar, Lancaster...).
 

ashishchandra

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vfone said:
You are looking for a 1% bandwidth of central frequency.
This is almost impossible to do using microstrip components. Microstrip filters generally have 10% up to 25% bandwidth of central frequency.
The only way to realize this filter is to use cavity topology. This can be tunable, but having big dimensions.

vfone can u provide me with any topology that can give me less than 1db insertion loss with 2% bandwidth of central frequency....
can u tell me what the order of dimension for that topology
help is very much appreciated.....
 

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