If you want to calculate Q factor from transmission you do not need PRBS signal, as you need the steady state simulation in frequency domain. Please refer this example https://optics.ansys.com/hc/en-us/articles/360042323794-Ring-resonator-using-INTERCONNECT-primitive-elements
One you get the transmission you can extract data and calculate Q from f/dleta_f where delta_f is the width of a resonance peak at f.
PRBS is for time domain simulation.
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