Designing a coupled microstrip transmission line using Sonnet

Hey,
I need some help with simulating a coupled microstrip transmission line on Sonnet. The two transmission lines however have different dimensions. One of them has a dimension of 22.5 microns x 90 microns and the other has a dimension of 3 microns x 40 microns. This design is to be used as a transformer for a millimetere wave circuit. I need to calculate the coupling factor of this design for various spacings between the transmission lines. Could someone tell me how to extract the self inductance and mutual inductance of this transformer using Sonnet?

I am confused because Sonnet offers three Inductance parameters: Inductance 1 is the effective inductance of a series RL network and gives Y11,Y22,Y33 and Y44. Inductance 2 is the series inductance between any pair of ports assuming a pi model. Inductance 3 is the series inductance between ports 1 and 2 with the other ports open circuited. How do I use these parameters to obtain the self inductance and mutual inductance parameters?

Thank you

The Sonnet inductance equations are based on different scenarios.

Inductance1 is the inductance seen from port 1 to ground. If other ports exist, they are shorted to ground.

Inductance2 is the series inductance seen between port 1 and port 2, with any influence from shunt elements removed. If other ports exist, they are shorted to ground.

Inductance3 is the series inductance seen between port 1 and port 2, with any influence from shunt elements removed. If other ports exist, they are open circuit. This equation is used for center tapped inductor, for example, with port 3 = center tap open.

For your transformer, you need other equations to get the k and L/M. Basically, you measure the input impedance into the transformer with (a) the secondary open and (b) short circuited. From there, you can calculate the values. I have an example on my website which includes the equations, assuming that you model the transformer as a two-port with +1/-1 and +2/-2. **broken link removed**

Hey,
Thank you for your help. However, I am still facing some issues with the design. The equation you provided me was giving me coupling factors of over 1. The frequency I am concerned with is 94 GHz. When I increased the spacing between the conductors, the coupling factor at this frequency was increasing. I have attached the sonnet file. Kindly give me your ideas and suggestions.

Thank you

Thanks for sending your project. I think your problem is from incorrect use of independent reference plane shift, which causes funny results. Those ref planes with different length are calibrated independently, unlike the normal box wall lines with shared reference. Now coupling between the de-embedded ref planes is NOT removed, and you see too much coupling in results (from DUT + feedlines).

You should contact Sonnet support to discuss alternative modelling. I will also point them to this thread, so that they learn about the misunderstanding with the independent ref plane feature.

volker_muehlhaus said:

Thanks for sending your project. I think your problem is from incorrect use of independent reference plane shift, which causes funny results. Those ref planes with different length are calibrated independently, unlike the normal box wall lines with shared reference. Now coupling between the de-embedded ref planes is NOT removed, and you see too much coupling in results (from DUT + feedlines).

You should contact Sonnet support to discuss alternative modelling. I will also point them to this thread, so that they learn about the misunderstanding with the independent ref plane feature.

Click to expand...

Thank you so much for your help!

Please see the attached project.

Changes:

1. Very important: "Analysis > Advanced Subsectioning > Polygon Edge Checking" set to more levels, so that the ground plane mesh is aligned with the lines above. By default, Sonnet only aligns the mesh with the metal on the level directly above and below. If the mesh is not properly aligned, you will get incorrect capacitance between line and ground plane.

2. Assigned port numbers +/-1 to one line, and +/-2 to the other.

3. Very important: Changed ref shift to shared, so that coupling in the feedlines is properly remove. Now the two line segments need to have the same length. This is for the coupling only, where we assume that coupling takes place where the lines are in parallel. To get the full inductance of the longer line, analyze that length seperately.

4. I think you model is somewhat over-simplified because we have only the straight segments, and the rest of the layout is missing. Current will only flow in closed loops, and the rest of the loop (which is not yet part of the model) might have some effect. I think you should include that in the Sonnet model as well.

The model is attached. With the transformer equation, I now get k=0.36 and N=0.73 for this model.

Thanks a lot for that. I see that you have changed the length of one of the microstrip transmission lines. I understand this is to use the shared reference planes and thus circumvent the feed line coupling issue. My objective however, was to couple two microstrip transmission lines of different lengths and widths. Is this possible?

npk_may said:

My objective however, was to couple two microstrip transmission lines of different lengths and widths. Is this possible?

Click to expand...

I understand, but wanted to calculate the coupling for two parallel lines first.
The coupling between segments of different length seems undefined to me,
regardsless of the Sonnet modelling issue. This would means a short wire where
current starts "out of nothing" and then magically disappears at the end of the line segment.
Even if you could simulate that, it has no physical meaning.

From many years experience in RFIC inductors:
To get realistic results, I think that you need to include your closed current loops,
not just a straight segment. Analyzing the magnetic coupling between straight segments of
different length seems strange to me.

The magnetic coupling depends on the area where the magnetic flux couples,
and the straight line segments with return path through the Sonnet box are probably
different from your real current path !? This will also change the coupling.

volker_muehlhaus said:

I understand, but wanted to calculate the coupling for two parallel lines first.
The coupling between segments of different length seems undefined to me,
regardsless of the Sonnet modelling issue. This would means a short wire where
current starts "out of nothing" and then magically disappears at the end of the line segment.
Even if you could simulate that, it has no physical meaning.

From many years experience in RFIC inductors:
To get realistic results, I think that you need to include your closed current loops,
not just a straight segment. Analyzing the magnetic coupling between straight segments of
different length seems strange to me.

The magnetic coupling depends on the area where the magnetic flux couples,
and the straight line segments with return path through the Sonnet box are probably
different from your real current path !? This will also change the coupling.

Click to expand...

Thank you once again for all your help!

Hey, could I bother you for some more help on this matter?

I am unable to understand how the equation was formed. Could you tell me what Z11 and Y11 are? Are these defined by you, or provided by Sonnet? I ask this because I see that Sonnet calls both Resistance 1 and Inductance 1 as Y11. I don't see Z11 being defined anywhere else but in the equation for Resistance 3.

I need to figure out the Self Inductance of each of the microstrip transmission lines as well as the mutual inductance. I know that I can find out the mutual inductance of the structure, once I know the self inductances of the two microstrip transmission lines and the coupling factor. Is the self inductance given by Inductance 1 (Y11/Y22)?

Thank you