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This page was created from content sent to us by Mert. Thanks for thinking of us! In his own words:
I was looking into designing a filter bank for an application. I came across your page on diplexers and was amazed by properties of singly-terminated Butterworth filters. I would like to share a design of mine with you, a cascaded N-way filter bank based on singly-terminated Butterworth filters. I would be glad if you can share & publish it, so that the community may benefit from it.
Singly terminated filters have the property that if the two diplexed arms are terminated in Z0, the imaginary parts of the reflection coefficients looking into their common ports are equal and opposite, so they cancel out and you are left with a common port that is matched to Z0. This is amazing
Mert's design divides 400 to 2000 GHz into 400 MHz passbands, using a cascade of diplexers. The crossover frequencies have 3dB response of adjacent filters. This type of multiplexer is called contiguous, as none of the overall band is rejected.
Here is the schematic. BTW, the Microwave Office project for this page is available in our download area. So is an electronic copy of Mattai Young and Jones filter book!
We wish to point out that the Microwave Office project contains an alternate topology, which is called a "waterfall" approach. Check it out on your own!
Each diplexer is made up of a pair of singly terminated, lumped-element nine-pole Butterworth filters, one is high pass and one is low pass. Here is the 1200 MHz version of the high-pass filter. You can obtain the "G" data from Matthai Young and Jones.
Here is the 1200 MHz version of the low-pass filter.
Finally, here is the response of this seven-port circuit. Bravo! Note the 3dB points where the diplexers equally split the signal at the transition frequencies.
Obviously, this ideal design will require a lot of attention to include all parasitic effects of the lumped elements. That lesson will have to wait for another day!
Reference
G. Matthai, E.M.T. Jones and L. Young, "Microwave Filters Impedance-Matching Networks and Coupling Structures".