Learning Forum

[Chi Zhang]

Hi,

I'm trying to simulate the external Q of a experimental set-up measuring the internal quality factor of a lumped element resonator (LER). I am following the method detailed in this paper (black-box quantisation, BBQ), which requires us to find the Y_11 of the system from a lumped port. The method has been validated for calculating Q_walls of the cavity of the mode of interest. To find external Q I have made everything inside the cavity lossless, and appended a 260 Ohm/sq impedance BC at the termination of the coax port. However, HFSS returns to me a negative Re(Y_11) (or equivalently, a negative Re(Z_11)) at the resonant frequency, thus predicting a negative quality factor (unphysical). I've attached a graph of the geometry and returned Re(Z_11) plot for reference. Any advice on how to proceed?

Thanks,

Chi Zhang

[Aymen Mzoughi]

You are trying to make a test for a structure from the paper you mentioned, but the pictures above explain little about the problem.
PEC is not the best practice to mimic superconductors. Resulting res. frequencies with PEC will be slightly higher than with a correct superconductor model.
However in general, this methodology can be successfully employed with HFSS:
https://www.researchgate.net/publication/222100999_Black-Box_Superconducting_Circuit_Quantization
https://journals.aps.org/prb/abstract/10.1103/PhysRevB.90.134504
https://www.kip.uni-heidelberg.de/Veroeffentlichungen/download/6168/pdf-6168.pdf