A GMSK modulator, using a Delta Sigma frequency discriminator-based synthesizer

This paper describes a new transmitter architecture suitable for wide-band GMSK modulation. The technique uses direct modulation of a Delta Sigma freq uency discriminator (Delta Sigma FD)-based synthesizer to produce the modul ated RF signal without any up-conversion. Digital equalization is used to e xtend the modulation data rate far beyond the synthesizer closed-loop BW. A prototype 1.9-GHz GSM transmitter was constructed consisting of a Delta Si gma FD-based synthesizer and a digital transmit filter. The synthesizer con sists of an 0.8-mum BiCMOS Delta Sigma FD chip, a digital signal processor FPGA, and an off-chip D/A converter, filter, and VCO. Measured results, usi ng 271-kbit/s GSM modulation, demonstrate data rates well in excess of the 30-kHz synthesizer closed-loop BW are possible with digital equalization. W ithout modulation, the synthesizer exhibits a -76-dBc spurious noise level and a close-in phase noise of -74 dBc/Hz.