A low-power active switched-capacitor loop filter for phase locked loops

We propose a low-power active switched-capacitor (SC) implementation of the loop filter for PLL applications. Control voltage ripples caused by current mismatches are eliminated. Sub-threshold inverters are employed as the active components of the filter resulting in very low circuit power consumption. Moreover, 1/f noise of the inverter amplifier is suppressed by the SC auto-zeroing. Transistor level simulation of a 2.4-GHz frequency synthesizer in a 0.18 μm technology is carried out to verify the proposed loop filter. With a 10 MHz input reference frequency, the sub-threshold inverter amplifier consumes about 110 uA with a 1.25 V power supply.