A 900 Mhz Ism Band Mash-12 Fractional-N Frequency Synthesizer For 5-Mbps Data Transmission

The design of a fractional-N PLL in the 900 MHz ISM band is presented to enable 5-Mbps data transmission for neuro application using GMSK modulation. The frac-N PLL consists of a phase frequency detector (PFD), charge pump (CP), loop filter, LC-VCO, and a prescaler. To minimize the CP current, the VCO control node capacitance is used to realize the on-chip loop filter's 3rd pole. Measured results of the MASH-12 frac-N synthesizer at 925 MHz show a phase noise of -104dBc/Hz at 22.5 KHz offset, in-band rms phase error of 0.7deg over 2.25 MHz bandwidth and out-of-band rms phase error of 2.3 deg. The measured phase noise spectrum correlates well with behavioral noise model predictions, including noise due to CP and PFD non-linearities. The chip was designed in 1.8V TSMC 0.18 mu m Mixed Signal and RF process.