EMRFD Message Archive 7740

7740 2012-09-04 16:58:52 Bill Carver Re[2]: [emrfd] A few questions on filters from chap 3 Message Date From Subject In a "top coupled" filter with small capacitor between parallel-tuned resonators the coupling is zero when the capacitor is zero. In a "bottom coupled" filter with large capacitor to ground between series resonators.the coupling goes to zero when the capacitor is "infinite". A side issue, but one you would find interesting, is that 0.13 pF coupling capacitor situation. Lets say you DO need 0.13 pF between two tuned circuits. If those parallel tuned circuits have, let us say, 50 pF tuning capacitors, one can split that into two pieces......just for purpose of talking, lets say 49 pF and 1 pF. Now draw the 1 pF capacitors to ground with the 0.13 pF between them. They form the greek letter "PI". I assume all undergraduate engineering students learn about "pi to T" and "T to pi" conversions, but perhaps never think to use them after they get out of school. You can convert that 1pF-0.13pF-1pF "PI" into a "T" of three capacitors that have precisely the same capacitance to ground, and the same coupling as the "PI". In this case it would be something slightly larger than 1 pF, then something on the order of 8 pF to ground, and another >1pF to the second tuned circuit. I'm sure you can find the mathematics on internet or a network textbook, under "PI to T transformations", or "T to PI transformations". It is an interesting thing to see. 73 - Bill >series tuned filters, how does one apply the coupling constants to a series >tuned filter? if i am shooting for a butterworth filter using top coupled >topology, i control the coupling with the inter-resonator capacitance. what >is the 'handle' for series tuned filters? my guess is that it would be the >mesh capacitors (going to the ground), but how does it affect the coupling? > >- farhan > > bill, i get it. thanks for pointing me in that direction. in a separate mail, i was corrected that regardless of the topology, the Q requirements of the reactive parts remain what they would be for the losses and the bandwidth. it is a 'smack my head' mistake that one makes all the time... let me try out some series tuned design tonight. i am just wrapping up another project as well. i hope to be able to post it soon. - farhan Whoa! Thanks a ton. I will smoke this stuff tonight! - Farhan