A problem in designing complementary diplexer in ADS

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.

When designing a diplexer you can NEVER design the HPF and LPF separately. This because always is an impedance termination relation between filters.
There is a rule of thumb in diplexer design, to do not place a capacitor from the joint point of the filters (antenna port) to the ground.

After the failure, now I know that I should design the two filters together. So now I leave what I did and redesign it. But how can i make the LP and HP filter complementary to each other? I used the same LP prototype and made the two filters have the same cutoff frequency. But when combined together, everything changed. Is there any other criteria for it?

Pls, fortunately I didn't place a shorted capacitor at the antenna port. But unfortunately, I still failed. :D

An Optimization Option on your simulator it will help designing a diplexer. In this way you can tune simultaneous both filters.
Also try to use Elliptical type filters (LP and HP), to get steeper rejection which would help getting good overall performance.

Hi

Like I know u must design LPF and HPF
but in order to combine then u must put λ/4 line.

David

David, you are right. At the beginning, I designed them seperately. When combining them together, I thought about adding two λ/4 lines, but the additional lines are pretty area consuming. Then I found out that a complementary diplexer can overcome the disadvantage. The LP and HP filters can work together well without any matching network. That's what I am proceeding, but I don't know how to design a complementary filter.

Vfone gave me an advice to use the optimization in ADS, thanks. It can be a way, but not a perfect method to it. There should be some theorem about the complementary filters.

how to design the third port in ADS for the two filter add together ?

The band width and pass band char are complimentary in LP and HP filters.
It is so happens that the RL of LP will cause the disturbance in HP and vice versa, when you connect both. This will not come in the case of stand alone.
Design of lambda/4 line will take care of narrow band but fails in wide band.