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# plz help in the design of combline bandpass filter

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#### sandeepsreeman

##### Member level 1
hi frnds i want to design a combline bandpass filter of order n=3 (so there should be 3 resonators).i want to design using transformed coupled lines.what abt n=0 and n=4 lines?(what are they called?) i studied that there are not resonators.how to find the length of those lines and i have to give 50 ohm feed to n=0 and n=4 lines . what is the value i have to take for Yai(resonator admittance)..i am getting problem to find the lumped capacitance value (in the design equations there is cotangent of electrical length is there iam using quarter wave resonator for design) plz help for design equations for combline and spacing betwenn the resonators ...... i referred to text book of microstrip filters for rf/ microwave applications by lancaster and hong ....but i couldnt understand how they designed...plz help

Tapping to the first and last resonators is much a much better choice and also saves spaces. Why do you want to use the transform coupled lines?

For the transform coupled lines, you can use the same length as your resonator. You need to control the space between your line and the first (last resonator) to obtain strong enough coupling. Normally you need to groud the line on the opposite side as your comeline to get strong enough coupling. The filter bandwidth is limited.

if i want to design using tapped line to design combline then what should be the length of tapped line and where i have to put the tap to the first resonator to get narrow band width.........another question is that if i want to design a filter using capacitive coupled series resonator(gap coupling) i found the value of capacitance(gap) and iam getting problem to determine the gap width how to determine the gap width if the gap capacitance is known

sandeepsreeman said:
if i want to design using tapped line to design combline then what should be the length of tapped line and where i have to put the tap to the first resonator to get narrow band width.........another question is that if i want to design a filter using capacitive coupled series resonator(gap coupling) i found the value of capacitance(gap) and iam getting problem to determine the gap width how to determine the gap width if the gap capacitance is known

You can use the same length as your other resonators, of cause you need to provide a frequency tuning screw to tune the frequency of each resonator to get the correct response. The tap position depends on your filter bandwidth, resonator length, as well as wire inductance that used for tapping. You can adjust the coupling strength by moving the tap position up and down (with respect to ground) to increase or decrease the coupling strength.

Your another question, are you referring to other filter other than combline filter? For combline filters, the couplings between resonators are always inductive.

hi when i want to design a combline bandpass what shuld the value i have to take for the resonator impedance ?whether it should be 50 ohm as of feedline or some other impedance when i referred to the text book of microstrip filters by lancaster he has taken resonator impedance arbitrarily but greater than 50ohm why?????????

There is a weak dependence of the unloaded Q on the resonator impedance. One can actually use any value. People normally use 60 to 100 ohm. For coaxial, 75 ohm gives the optimum unloaded Q. It really doesn't matter which impedance one use.

what equation did you use to find the lengths and spacings for your filter. Also is there any way of finding out what N would be best for your frequency range

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