This paper presents a sample & hold pulse envelope synthesis method and shows that only 5 (resp. 7) samples are necessary to respect the sidelobes rejection in adjacent channels of 18dB (resp. 20dB) required by the IEEE 802.15.4 (resp. 802.15.6) standard if a sample & hold Gaussian envelope is used. These required numbers of samples, which enable complexity reduction of IR-UWB emitters for IEEE standards, are validated using measurement results of a pulse synthesizer presented in this paper since a sidelobes rejection in adjacent channels of 21.8dB (resp. 21.4dB) is obtained when transmitting in the mandatory low band of the IEEE 802.15.4 (resp. 802.15.6) standard.