In this new entry of the blog, I will share the results of several simulations that prove there is some sort of bug in the equations of the microstrip coupled lines model in QUCS Studio, that produces wrong results in the simulations, once several parameters of the microstrip substrate are set in a determined manner.
This bug has been discovered by Margeride48. She brought to my attention several strange results simulating microstrip coupled lines. She wanted me to try to reproduce her results, just to confirm the wrong ones that she was getting from the simulator.
The following lines explain the anomaly in her words:
Focusing on coupled microstrips lines, have you tried thinner dielectrics ?
I'm simulating a 0.254mm I-Tera MT RF material.
S-parameter simulation values are going suddenly totally wrong when thickness is lower than 0.44 mm (eg 0.435mm).
Same for the Coupled Microstrip Line Calculator !
Impedances, losses, etc, become suddenly wrong.
On the opposite, the 'single' Microstrip Line Calculator leads to good results for 0.254mm thickness.
An EM simulation performed on the auto-generated PCB give good results with 0.254mm thickness for coupled microstrips.
We made a little investigation and I think that the best way of describing the findings is to consult the document linked to this entry that summarizes the results we got.