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I am trying to design a complementary diplexer using LP and HP filters combined parallelly. I have achieved LP and HP seperately according to the basic filter design knowledge. But when I combine them together, the characteristics I have achieved for each filters become unacceptable. I tried to figure it out, but....

So, could anyone here help me about this? Thanks in advance. Examples or reference book are appreciated.

For a diplexer the LP and HP filters are designed differently than ordinary filters that are designed to work in a system Zo (a constant Zo). At the common node of a diplexer each filter must work properly while looking into the impedance of the other filter that is also in parallel with the system impedance. So what this means is that a low pass and high-pass filter pair that was originally designed for some system impedance, Zo, cannot always be connected in a diplexer configuration with successful results. Technically, you have to use a different low pass prototype table. I am not absolutely certain, but I believe that table uses a filter working into a constant voltage at one end. I'm not sure what you're after exactly, but you should be able to find simple lumped element diplexers off the internet fairly easily. If you want I can try to find something for you or even try to find the right table in the Zverev book.

Thank you very much! You are so kind! Now I changed my project, but anyway, THX very much!! Wish you good luck!)

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