Simulation of two coupled lines with MWS

Hi there,

I'm a student and I have to simulate a two-conductor microstrip line with ground plane with CST MWST 5.0.
I already created the model of the microstrip, but there are some questions left:

1. Each line is connected with 50Ω to the ground plane (beginning and end). I tried to integrate them as lumped element connecting the line and the ground plane, but MWS complains that they are inside the PEC material. What is the right way to do this?

2. One of the lines is the quiet line and is influenced by its neighbour with a signal source. So one side of this line has to contain the 50Ω resistor and a port. Which type of port do I choose to stimulate the microstrip?

3. I also have to take a look at NEXT and FEXT. Are these results of an excitation signal I can view after solving? How do I view the results of the quiet line?

If someone could help me with these questions, I'd be very thankful.
Norman

Hi Norman,

1. Make sure your model is'nt embedded inside PEC background.
Go to Solve->background materials->choose Normal.

2.Another option is that you have Perfect-E boundaries touching a lumped element.
See 1 and add some space to the lumped element side.

Plz post your model.mod and model.par file here so we can take a look.

Goodluck
P.

Here's the simulation model I created. Are the lines and the lumped elements still connected if I add some space between ?
Hoping for some hints....

Thanks
Norman

Norman:

What are you trying to do with this example?
If you are worried about getting proper terminating impedances, then you should use the multi-pin ports. You can't put a lumped element termination in the same plane as the field port. In fact, I think MWS requires a certain number of mesh lines in from the field port definition to be constant before other things are added to the cross-section.

You can do one of two things:
1. Add space between the ends of your lines and the work space, and replace your lumped elements with discrete ports. The discrete ports can have internal impedances that you define so you have the termination you want.

2. Use a multi-pin field port so you can define two unique signals on the two lines that cross each port boundary.

I don't like the discrete port idea at high frequencies because I think they will interact and add error to your results.

This leads to another question: Why are you using a full 3D simulator for such a simple planar problem? You can get the job done a lot faster, with less headache and with better accuracy with a planar Moment Method simulator. This problem looks like a simple one with Sonnet Lite. There may be other free planar MoM simulators out there as well. I would really suggest you use the right tool for the job.

Thanks!

--M

At first thanks for your answer!
My aim is to simulate an example my professor gave to me. I HAVE to do it in MWS (not that I want to).
But I don't know how to use these discrete ports: If I add the ports right behind the line, I get a "Port inside PEC"-error from the solver even if I add space to it.
How far away are these ports still working?
I connected the port from the line to the ground plane, right?

I have to admit I "love" MWS!

Norman:

OK, I understand the problem.

First of all, you're getting the "Port inside PEC" error because your mesh controls are still too restrictive. Change the Global Meshing Settings to allow a smallest cell size of something like 0.02 (like 3 cells or so across your dielectric in the Z-direction). You might also need to change the Lines/wavelength and the second setting (forgot what it's called) so that you don't see the "blue blobs" in the mesh view window.

What is happening is that since your cell size is too large, it's not resolving the difference between your microstrip lines and the ground plane below.

Try setting Zmin and Zmax boundaries to "Open Add Space" and changing your lumped elements to discrete ports.

Remember that you still have discontinuities with this at the ends of your lines, where the current turns the corner for the discrete ports. For this reason, I'd favor the multi-pin ports since they don't have this discontinuity that might screw up your sensitive NEXT and FEXT measurements. If you have time, look over the multi-pin ports; they are better suited for this kind of thing since they don't introduce extra discontinuities that will ultimately lower your return loss for each line.

I know I'm beating the wrong drum here, but this problem is *much* easier to handle with a shielded MoM code. Because of your E boundaries, this problem is a nearly perfect example for Sonnet Lite, and Sonnet Lite would probably converge to an accurate result more quickly (and with a lot less work). I think you'll want to turn on mesh adaption in Microwave Studio to make sure that you are getting a fine enough mesh, especially if NEXT and FEXT values you are trying to resolve are on the order of -100 or even -70 dB. This is just a question of your professor figuring out which tool is optimal for what kind of homework problem. But I know that's not your problem.

-Max

Added after 34 minutes:

Norman:

One other comment. If you want to get NEXT and FEXT values for this coupled line pair, you had better use the default Gaussian signal type (excites all frequencies in your simulation band equally) rather than the rectangular pulse type. The rectangular pulse will show you the time domain response only. I guess if you only want to see actual NEXT or FEXT in time domain (not the usual curves over bandwidth), then you can use the rectangular excitation. However, be sure that you make your mesh fine enough to have time simulation steps small enough to resolve the time domain waveform properly.

I guess I've probably said enough already. Good luck with your studies.

--Max

maxwellian, thanks for your help! that did the trick. I already thought it had something to do with the grid settings but I didn't know what to change.
I chose the rectangular signal because it's given in a presentation like the whole microstrip that I have to simulate.

I finally have some other questions to the model of the 2 coupled lines:

1. How do I find out up to which frequency (Fmax) I have to simulate? By rule of thumb (0.5/0.25ns) I chose 3 GHz but MWS is still complaining for the rise time of 0.25ns is.

2. Is it possible to view the spectrum of each port?

3. I have to extract a pSpice model. Is transmission line or model order reduction better? I tried MOR but MWS said something like S-parameters show active behaviour... what's wrong?

I hope thats not too much to ask for...
Norman