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A single-chip baseband waveform generator CMOS-LSI has been fabricated for use in the quadrature-type GMSK modulator, which is suitable for application to digital mobile communications. The LSI employs digital signal processing in realising such functions as Gaussian baseband filtering and phase integration. Digital/analogue convertors and auxiliary circuits, such as an internal-clock generator and carrier frequency adjuster, are also integrated onto the same chip. The LSI operates well below a bit rate of 110 Kbit/s when the power supply voltage is 5 V.