A simple CPS line simulation

Hi,

I am trying to simulate a simple coupled microstrip line with NO ground. The dimesions of the microstrip lines: w=0.25um,thickness= 0.05um, spacing=0.32um(only for simulation), height of the substrate is 0.1um. I wanted to know the type pf port to use . However I am using waveport, do i need to enclose the structure in air-box. Please let me know how can I define an integration line,i.e. should it be the whole length as the port or from the bottom of the port to the condcutors?..

Thanks for help

This is similar to your previous problem only that you don’t have a ground now.
Then the integration line should not go to the bottom of the port, but it should simply connect the two conductors. The port should extend lower than the edge of the dielectric into the air.
If your structure is in air you need to put an airbox around and radiation boundaries.

Thanks Fekete..... as far I understand from your reply.... the integration line should be connected between two conductors, is that right?......Also I am using similar port at the other end of the strips, so should I place the same direction of integration line or should be in opposite direction to the first one?.

Yes, between the conductors. I would connect the centers of the connectors if the metal in not perfect conductor, if it is PEC then it can be between some edge points.
The direction only changes the phase of the S21 that is calculated. Better if you keep the same direction, then at the limit of 0 length you will have same phase, as normal, not opposite phase at the output.

Hi fekete,

I tried simulating the structure, but I am getting some weird results. Could you please check my project and comment on it. Your help will be appreciated.Thanks

Where is your project? Did you attach some files? I don’t see them.

Hi fekete,

I already did.....i am attaching again.

The projects has numerous errors
I mentioned earlier to use radiation boundary conditions everywhere on the air box, but you did not put any boundary conditions (by default they are perfect conductors)
The ports are too low, top edge touches the two strips shorting them (port edges are PEC)
Increase the size of the ports, make them equal to the face of the airbox in your case.
However, the airbox also has to be bigger, there is no air below the substrate, you should have some, in your case roughly as much as above since the substrate is thin (or is there another dielectric underneath ?)
You did not associate a useful material to the strips, they are vacuum …
You should use symmetry boundary condition to reduce the computation time.


there are some other strange issues with your problem
You are simulating at 80 THz, that is infrared “light”, the wavelength is 4 microns. The “light” used in fiber optics has 1.5 microns – 200 THz. Metals at these frequencies behave very differently than the simple conductivity model. They have a complex dielectric constant with the imaginary part nothing near the value given by low frequency conductivity. You have to find those material parameters from the literature.
In fact, dielectric waveguides are used in that range, except in some research projects.
You didn’t answer to my question about how do you launch this light into your structure.
How is this very this structure supported? Is it on some other dielectric?

HI Fekete,

There is no other dielectric, as far I know. The light source would be a IR source. I am worrying about correct simulation of the structure. I have to worry about the experimental part later. This structure will have a complex load at one end. Actually i need to know teh complex impedance of the structure sitting on the coplanar strip lines, which will then be matched to the impedance of the antenna.Since I am only worried about finding the impedance of the load on the CPS, I am trying to simulate a simple CPS structure , to see if I am getting reasonable results. Is it possible to use lumped port instead between the coplanar lines ?. I can send you the copy of reference paper.

I have seen a few papers using metals, transmission lines and antennas for IR, usually for detectors. However, a reference that is specific to your problem would be useful.

hi

The configuration in the paper has a thin SiO2 on top of a Si wafer. I think you also have another dielectric beneath your 0.1 micron dielectric (something like that cannot be free-standing), and this changes your simulation.
Anyway, I modified your original design and now I see reasonable results, nice port mode as expected and relatively flat S parameters, also as expected.

Attached you have the modified hfss file.
I changed the dimensions into variables for easier redesign (Go to the HFSS-> Design Properties menu).
You can see the characteristic impedance of the ports (transmission line).
I did not put the symmetry condition so you can add whatever structure you have after this. You can use a lumped port between the lines, but waveport is preferable.
I hope the file gets to you will all the details (boundary conditions etc).

Hi Fekete,

Thank you for your help. I however have one more question, if i use lumped port then, I define lumped port in between the two strips is that right ?. I also observed that by changing the width of the port the E-field changes. How will i know if I am using a right size port in case of lumped port ?.

Lumped port is a worse approximation than a waveport, and in your case it is not recommended. While I don’t understand exactly what kind of changes you see, and what you mean by width (the vertical size, right?, because the horizontal is equal to the gap)
A good choice for your case would be a small rectangle of the height of the conductors, occupying the gap between the conductors. Then the integration line horizontally at the middle of this.

Hi Fekete,

I tried simulating the modified HFSS design you sent me earlier. However, I tried to simulate the design from 50Thz to 110 Thz, fast sweep and checked the smith chart for s11. I am not sure if i get the right result. could you please look into it and advise me.

How is the smith Chart?
Try interpolatory sweep, and do two separate simulations, for 50 to 80 and 80 to 110 THz, the range is too wide for one simulation and one mesh.

Hi,

I wanted to know if i use Ground plane say of gold in the above given structure. How can I define my ports. I tried simulating the structure but I see that the Smith chart gives me no response when I try to simulate the grounded CPS.

I wnat to know how can the ports be defined.

I obtained the error: Pot Refinement: process abc3d died unexpectedly

have you properly displayed the sequence of normalized impedance, admittance or reflection coefficient in the circle of unity radius? i don't think you need anything rather than the smith chart.

Yes,

I get the message that simulation completed with execution error on server.
Port Refinementrocess abc3d died unexpectedly.

I want to know how to define a port if is is grounded CPS.

You should delete the ports used in the other case and redefine them with the difference that the rectangle should go down only until the lower edge of the ground metal, and that now you have two modes.