differential ports hfss

I am sorry to bother you with my questions. My width of coplanar strips is w=0.25um, seperated by s=0.32um and the substrate is h=0.1um thick. When defining waveport, in HFSS manual it is described to consider the width of the waveport as approx 5*w, and length as >4*h. So the 2 ports practically would touch each other. How can I define the port?.

what kind of frequency are you talking about? Type of applications? A 0.1um thick substrate is sooooo strange. Are you sure the specification is correct? It seems to me this is not a "practical" problem.

Anycase, I would simply go ahead and get some result from HFSS and see if it is reasonable. To define the waveport, there is no need to worry about "two ports too close to each other, because only the cross section is relevant.

hi Loucy,

yes the specs are right. I am looking into terahz frequencies....Please let me know if teh simulation works?....aslo the boundary conditions to be used....

Added after 3 minutes:

hi Boucy,

i forgot to add.... i am having a complex impedance load sitting on top of the coplanar strips. I need to find teh impedance of the load.

09
Is not the length, but the height >4h
They touch laterally, right?
If I understand correctly, you are trying to use separate waveports for the left and right stripline. If that is the case, you should use one port for the two ? and then at the other end of the structure maybe another port, again for both lines.

where exactly is that load ? at the end of the lines or in the middle of the structure?
You need to find the impedance of the load or the input impedance of the structure? Seems to me that you know the load? what kind of (lumped element) load you can find at THz frequencies?
Do be so stingy with the details, we don?t know what are you trying to do; sometimes a small detail can completely change everything

Hi,

If the interaction between the lines are critical define a bigger wave port to cover both of them, and draw two terminal lines (change to Driven Terminal solution type).

Please increase port accuracy to 0.5% if the port is big.

Itai

hi Fekete,

I am trying to simulate coplanar strip linesof length 1um, thickness 0.05um, width=0.25um,spacing between two coplanar lines=0.32um, on substrate of thickness 0.1um witha ground plane. I put another material of length= 1um ,width=0.82um, thickness of = 1um, on top of the two microstrip lines. I was using lumped port between the two strip lines to simulate my structure but no reasonable results are obtained. I am trying to find the complex impedance of the structure sitting on the coplanar lines or the impedance of the structure as a whole. I am using only one port to simulate the structure, the other end of the microstrip line is left unassigned.
i dont know the impedance of the structure, I am trying to find it. All i have is the dimension of the structure sitting on the coplanar lines.

Hi 009,

Aha, so there is a ground plane ? important detail, these are then the so-called Grounded Coplanar Strips GCS.
You should probably model the metals (including the ground) as lossy metal, maybe a dispersive model (Lorentz)
Also, these metals, due to fabrication issues, have a very rough surface at your scale.
How do you excite this structure experimentally?
Note that your structure admits two quasi-TEM modes because you have three metals. When you excite with a lumped port between the two lines, you implicitly assume that only the odd mode it is present. However the discontinuity generates even mode components too.
Best to put a waveport, you will have two modes, or two terminals, and I presume that experimentally you excite the odd mode.
Now I see that you don?t actually need to put a RLC in the simulation. Simply put what material dimensions and properties are there, and after simulation you want to see the impedance matrix (basically the input impedance).
In your case you should use differential port, see the ansoft documentation on that ? it?s quite a lot to read. You seem to have a physics background and not high-frequency EE, so you will have to suffer.
When you say the other end is left unassigned, I hope your have put radiation boundary conditions or pml (otherwise you will have reflections that change everything) ? that means you have somewhat an open circuit (which also give reflections, but probably that is the experimental situation)

Hi fekete,

i am a new user of HFSS, and the documentation doesnt actually supports new users, thats the reason why I see lot of HFSS questions being asked in the forum. I do have a electrical background having my bachelors and masters +...,but this is not the area I concentrated on before. I am learning new things and my microwave/RF fundamentals needs to be brushed up and hope you and other guys in the forum will help me in this process.

If the wave impedance of the port is all you need, just solve the port only.

Otherwise you would need to assign another waveport at the other end, PML or radiation boundary won't give you accurate result because the length is so small.

It would be interesting to know how you measure this thing.

Loucy

A port at the end would assume that this line is matched.
I assumed that it is left ?open? in the real devices, since 009 is not interested in S21, only in the input impedance of this structure with that given length
If he actually wants the characteristic impedance, then indeed needs only port solution

what if i am terminating the structure with an antenna?.... how am i going to define the port and please could you explain to me in detail.... as I am a new user of HFSS.
Thank You

See?, again another detail that changes a lot. Loucy and I were trying to guess what you have there. Unfortunately you are not the only one who doesn?t give enough detail, and then most of them are disappointed that they are not getting useful answers.
Anyway, you should put your antenna structure in the simulation model. What kind of antenna is?
Radiation boundary conditions everywhere. except at the bottom where you can leave PEC (if your ground is thick enough) after you define a lossy metal as ground.
How thick is this ground metal?
Define the port after these boundaries. A rectangle of dimensions you already know from documentation, include both lines and the ground metal. I usually run a port only simulation at the beginning and check the port modes to see that nothing strange happens, a geometrical error or dimensions not chosen well.
If I understood correctly the way you excite this structure, you need a differential port.
To be sure, you should explain how is this structure excited, I assume you have some kind of a semiconductor device on the same substrate. What substrate is anyway?
What is the dielectric on top?