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The paper presents the use of the pulse position modulation (PPM) index, β, and pulse doublets for spectral line suppression on PPM time hopping impulse radio (TH-IR) ultra wideband (UWB) systems. We define a pulse doublet as a pulse waveform formed by two identical base pulses, with opposite polarity, separated by a fixed time, TD. It is shown that, by properly choosing the values of β and TD, a good number of the spectral lines present on the power spectral density (PSD) can be effectively eliminated. In the example presented, the time hopping code is not considered as it also can help to improve the PSD.