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Low pass filter using CST simulation

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Shahendaeid

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Microstrip filter simulation

I was trying to make simulation for LPF example in Pozar .but during the simulation I found these errors

[1]At least one propagating mode is not considered at port 1.
[2]Mesh adaptation terminated because the maximum number of passes is reached.

I could not solve them so I could not get the result as found in the book
Awaiting replies,

Regards
 

Re: Microstrip filter simulation

what software are you using?
 

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Re: Microstrip filter simulation

I used CST .
 

I tried to simulate the LPF as found in the attached photo but the result is not true at all.as the errors I get as mentioned in the previous post.I want to know if the feeding of the filter using wave guide has specific dimension or not as my friend told me that the error may be happened because of the using of wave guide port.the result i get as shown in attachement
Note: I used CST simulation
 

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hi,
the results looks bad and doesnt make sense. i agree with your friend. please use discrete port between the input pin and the ground. if there is no ground underneath the input pin, you may have to create an artificial ground underneath input pin. this is just to make discrete port connected to the ground. you have to do the same for waveguide port as well.
 

I could not understand artificial ground underneath.
 

Re: Microstrip filter simulation

Shahendaeid said:
[1]At least one propagating mode is not considered at port 1.
Click to expand...

Maybe your waveguide port is too large. The height should be between 3-5 times the substrate thickness, the width should be between 4-6 times the microstrip width. You can check your waveguide, by choosing "Properties" in the menu if you right-click on your waveguide (i.e. in the excitation source tab) and press "Calculate". In the case of a microstrip line you should get "QTEM" as mode. I advice you not to use discrete ports for filters but waveguide ports because of an increased accuracy. And also check your background and boundary conditions.

Shahendaeid said:
[2]Mesh adaptation terminated because the maximum number of passes is reached.
Click to expand...

The mesh adaption did not reach the predefined accuracy. You can set the numer of passes in the setup, but if you choose a correct excitation and sufficient start mesh density, mesh adaption should be accomplished after maybe 5-8 passes.

Btw, I saw your pcb setup, remove the pcb till the ends of the transmission lines and place your waveguide ports. Also ensure your pcb backside has a metal plane as ground.
 

Shahendaeid said:
I could not understand artificial ground underneath.
Click to expand...

artificial ground means, in actual design you dont have ground underneath input and output pin. however, since u need to insert port1 and port2 respectively so you have put a ground pad underneath. this is applicable for waveguide port and also discrete port.
 

can you post the pic of your CST layout in here? that will help us to figure out exactlty where you went wrong rather than guessing. please post clear view of port1 and port2.
 

pragash said:
can you post the pic of your CST layout in here? that will help us to figure out exactlty where you went wrong rather than guessing. please post clear view of port1 and port2.
Click to expand...

filter.png

the layout of the filter as shown in figure and i used coaxial waveguide port
 

Ports are apparently unconnected to the filter structure.
 

you meant that the ports must be connected to the beginning of the filter not from the beginning of substrate
 

Shahendaeid said:
you meant that the ports must be connected to the beginning of the filter not from the beginning of substrate
Click to expand...

yes. ports must be connected to beginning of filter and not substrate. excitation/RF input of real filter will be on the conducted and not substrate!
 

I connect the ports to the filter but the result is still not good as shown in figure below
 

Do you still use discrete ports? If so, and you don't want to use more accurate waveguide ports, place the discrete ports at the edges of your microstrip trace! Also the filter looks different compared to the pic of your third post. Do you have a backside metallization?
 

its because you dont have another conductor underneath. for your case, this another conductor will be ground plane. any ports in CST must be connected between two conductors.

as i mentioned before, you need to have artificial ground plane underneath to connect another side of the port. if your design doesnt have ground plane underneath, you can have this artificial ground plane as far as possible from the design. this will help you to get accurate results as implementation/PCB board.
 

pragash said:
you can have this artificial ground plane as far as possible from the design. this will help you to get accurate results as implementation/PCB board.
Click to expand...

In the case of discrete ports you'll get wrong results because of port parasitics!
 

johnjoe said:
In the case of discrete ports you'll get wrong results because of port parasitics!
Click to expand...

Without another conductor connecting the port, you won't be getting a result at all. What you will get is garbage like what have been shown here. results with parasitics is better than a result with garbage!
 

I did not use discrete ports.and I have backside metallization as the design contains three layer
1-ground was Perfect electrical conductor
2-substrate was made from material that epsilon=4.5
3-the filter that made from perfect electrical conductor
 

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