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-m CMOS process that consumes 27 mW. Key circuits on the custom IC are an onchip loop filter that requires no tuning or external components, a digital MASH 6-1 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.

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A fully integrated, low phase noise, dual-band voltage controlled oscillator (VCO) utilizing a novel tuning scheme is reported. Coarse digital tuning is achieved using MOSFETs and fine analog tuning utilizes PN varactors. The measured phase noise is -95dBc/Hz max at 3GHz +25kHz over the whole tuning bandwidth (f12.5%), and the power dissipated is 22.5mW. To our knowledge, these are the best results published so far.

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