The paper presents methodology for analyzing compliance of Impulse Radio Ultra-Wideband (IR-UWB) communications systems with regulatory limits specified by US Federal Communications Commission (FCC). First, discrete spectral component that reduce Effective Isotropic Radiated Power (EIRP) in IR-UWB spectrum are analyzed in an exemplary IRUWB system. After that, a minimal Pulse Repetition Frequency (PRF) in an IR-UWB system at which EIRP spectrum level does not have to be reduced in order to meet peek power regulatory constrain is derived for two different modulations; namely, On-Off Keying (OOK) and Binary Phase Shift Keying (BPSK).