A fractional-N synthesizer for software-defined radio with reduced level of spurious tones

We present an integrated fractional-N frequency synthesizer providing in-phase / quadrature phase signal over the frequency bands 0.6-4.6 GHz, 5-7 GHz, 10-14 GHz, and in-phase signal over 20-28 GHz for software-defined radio applications. The use of two parallel charge pumps for VCO tuning results in a fairly constant in-band phase noise over a wide range. Phase noise in fractional mode is -100 dBc/Hz and -122 dBc/Hz at 10 kHz and 1 MHz offset, respectively, for an output frequency of 3.644 GHz. The near-integer spurs related to the substrate modulation by the VCO were reduced to below -60 dBc by shielding the reference input buffer. Fabricated in a 0.25 μm SiGe-BiCMOS process, the synthesizer draws 45 mA from a 5 V supply and 140 mA from a 3 V supply and occupies an area of 4.8 mm².