hello,
i am a student and we got a hf task for a lab project. we have to
built a low pass filter (-3dB @ 500MHz, 3th order) in microstrip,
coupled lines and interdigital. the first two we have to build and
measure, the 3th one is to calculate only. i know the last two filters
are band pass, but we should design something with highest brakepoint
at 500MHz.

now my question:
we managed to calculate the microstrip filter so that one is done and
simulated. what botters us is that we didn't get ANY sort of a course
(we have to find anything on our own) and we have no clue about the
coupled lines and even less about the interdigital filter.
there is very very little useful information to find on the web so
please can someone help me here by giving some basic
methodology/formulas for coulpled lines or interdigital filters. maybe
someone has a few pdf's or other info on his harddisk he/she could
send me?

thanks in advance
lenx