Hello Micasa,
I hope I understood your question correctly,
go to "Transient Solver" -> "Specials..." -> "Solver".
Under the frequency settings section you'll find the number of samples CST uses,
Increase it and see what happens.
Good luck,
P.
Added after 29 minutes:
w8 a second.... now I see your plots...
I have'nt tried it myself but :
go to the "Port Signal" on the 1D result folder.
Export the o1,1 signal to ASCII file and try using FFT on this signal.
PLZ tell me if it works...
P.