A 27-mW CMOS fractional-N synthesizer using digital compensation for 2.5-Mb/s GFSK modulation

A digital compensation method and key circuits are presented that allow fractional-N synthesizers to be modulated at data rates greatly exceeding their bandwidth. Using this technique, a 1.8-GHz transmitter capable of digital frequency modulation at 2.5 Mb/s can be achieved with only two components: a frequency synthesizer and a digital transmit filter. A prototype transmitter was constructed to provide proof of concept of the method; its primary component is a custom fractional-N synthesizer fabricated in a 0.6-μm CMOS process that consumes 27 mW. Key circuits on the custom IC are an on-chip loop filter that requires no tuning or external components, a digital MASH Σ-Δ modulator that achieves low power operation through pipelining, and an asynchronous, 64-modulus divider (prescaler). Measurements from the prototype indicate that it meets performance requirements of the digital enhanced cordless telecommunications (DECT) standard