driven terminal and modal base solution type

terminal modal s parameter

Hi,
I simulate a transition sturcture from cpw to microstrip line. I start from an assumption that the cpw's gound is infinite. The cpw gap is about 7.2um and the gound width is about 200um.Then the simulation result is perfect. But as I reduce the ground width to 150um, the result varies dramatically by using driven terminal solution type. In gerneral, the filed is very small far from the signal line. Therfore, this is unusunal. But using modal based solution type, the result is almost the same.In the setup, I just set the mode number to 1 ,but frequency is simulated up to 80GHz. I have no idea if the mode number equaled to 1 is ok in my case.And If i just want to know the s parameter of my strucutre, which solution type is correct?If it is modal based,are there any guides about the setup of mode number and it can give a good simuation result that may match to the measurement result?

Added after 26 minutes:

Sorry it is simuated by HFSS

Added after 52 seconds:

Sorry it is simuated by HFSS

the number of modes is related to the propagating modes that will propagate in your structure, if you are operating at such a high frequency then you better increase the number of modes to obtain better solution.

since ther emight be higher order modes excited at the transition and HFSS did not include it in the simulation.

Best Regards,
Adel

For analysis of a single-ended transmission line, I believe one mode should be sufficient presuming "quasi-TEM" qualifies as a single mode. For 2 conductor TL's (e.g. a differential pair), then 2 modes should be included; for 3 conductors, 3 modes, etc.

I think this is the case if you are conisdering that TEM modes only will propagate, however in many cases higher order modes may either propagate (in high frequencies) or be come evanescent in this case you should put the ports far away so that all of them have decayed in order not to affect the accuracy of the solution

Yes, I agree on adel-48's point. The higher-order modes are generated at discontinuities and so it goes that to have high modal purity at your ports, you want to avoid having discontinuities there...

According to the documentation for Ansoft Designer on the subject:

Modes, Reflections, and Propagation

It is possible for a current solution generated by an excitation signal at one specific mode to contain reflections of higher order modes which arise due to discontinuities in the high frequency structure. If these higher order modes are reflected back to the excitation port or transmitted onto another port, the S-parameters associated with these modes should be calculated.

If the higher order mode decays before reaching any port — either because of attenuation due to losses or because it is a non-propagating evanescent mode — there is no need to obtain the S-parameters for that mode. Therefore, one way to avoid the need for computing the S-parameters for a higher order mode is to include a length of transmission line in the geometric model that is long enough for the higher order mode to decay.

For best results, include a small length of the transmission line that is connected to each port in the model of the structure. For example, if one port of a structure is connected to a microstrip line, then the portion of the geometric model leading up to the port must have the cross-section of the microstrip line.