An eighth order active-RC filter for low-IF and zero-IF DVB tuner applications is presented,which is implemented in Butterworth biquad structure.An automatic frequency tuning circuit is introduced to compensate the cut-off frequency variation using a 6-bit switched-capacitor array.Switched-resistor arrays are adopted to cover different cut-off frequencies in low-IF and zero-IF modes.Measurement results show that precise cut-off frequencies at 2.5,3,3.5 and 4 MHz in zero-IF mode,5,6,7 and 8 MHz in low-IF mode can be achieved,60 dB frequency attenuation can be obtained at 20 MHz,and the in-band group delay agrees well with the simulation.Two-tone testing shows the in-band IM3 achieves-52 dB and the out-band IM3 achieves-55 dB with-11 dBm input power.This proposed filter circuit,fabricated in a SMIC 0.18μm CMOS process,consumes 4 mA current with 1.8 V power supply.
A fully-differential charge pump(FDCP)with perfect current matching and low output current noise is realized for phase-locked loops(PLLs).An easily stable common-mode feedback(CMFB)circuit which can handle high input voltage swing is proposed.Current mismatch and current noise contribution from the CMFB circuit is minimized.In order to optimize PLL phase noise,the output current noise of the FDCP is analyzed in detail and calculated with the sampling principle.The calculation result agrees well with the simulation.Based on the noise analysis,many methods to lower output current noise of the FDCP are discussed.The fully-differential charge pump is integrated into a 1–2 GHz frequency synthesizer and fabricated in an SMIC CMOS 0.18μm process.The measured output reference spur is–64 dBc to–69 dBc.The in-band and out-band phase noise is–95 dBc/Hz at 3 kHz frequency offset and–123 dBc/Hz at 1 MHz frequency offset respectively.
