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.