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A ΣΔ fractional-N frequency synthesizer targeting WCDMA receiver specifications is presented. Through spurs compensation and linearization techniques, the PLL bandwidth is significantly extended with only a slight increase in the integrated phase noise. In a 0.18-μm standard digital CMOS technology a fully integrated prototype with 2.1-GHz output frequency and 35 Hz resolution has an area of 3.4 mm2 PADs included, and it consumes 28 mW. With a 3-dB closed-loop bandwidth of 700 kHz, the settling time is only 7 μs. The integrated phase noise plus spurs is -45 dBc for the first WCDMA channel (1 kHz to 1.94 MHz) and -65 dBc for the second channel (2.5 to 6.34 MHz) with a worst case in-band (unfiltered) fractional spur of -60 dBc. Given the extremely large bandwidth, the synthesizer could be used also for TX direct modulation over a broad band. The choice of such a large bandwidth, however, still limits the spur performance. A slightly smaller bandwidth would fulfill WCDMA requirements. This has been shown in a second prototype, using the same architecture but employing an external loop filter and VCO for greater flexibility and ease of testing.
Date of Publication: Sept. 2004