Identification help

I am working on identification project in college. I have the system with time constant of 100 sec and sampling time is 2 sec.
I want to design input PRBS(Psuedo Random Binary Signal) for experiment. I am using MATLAB iddata() command for this. It requires the frequecy range for which PRBS is to be designed.
I am confused what frequency range should i use?? Please help me.
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