A GMSK modulator using a ΔΣ frequencydiscriminator-based synthesizer

摘要

This paper describes a new transmitter architecture suitable fornwideband GMSK modulation. The technique uses direct modulation ofnΔΣ frequency discriminator (ΔΣFD)-basednsynthesizer to produce the modulated RF signal without anynup-conversion. Digital equalization is used to extend the modulationndata rate far beyond the synthesizer closed-loop BW. A prototype 1.9-GHznGSM transmitter was constructed consisting of a ΔΣFD-basednsynthesizer and a digital transmit filter. The synthesizer consists ofnan 0.8-Μm BiCMOS ΔΣFD chip, a digital signal processornFPGA, and an off-chip D/A converter, filter, and VCO. Measured results,nusing 271-kbit/s GSM modulation, demonstrate data rates well in excessnof the 30-kHz synthesizer closed-loop BW are possible with digitalnequalization. Without modulation, the synthesizer exhibits a -76-dBcnspurious noise level and a close-in phase noise of -74 dBc/Hz