Digitally augmented analog phase locked loop with accurate bandwidth

Abstract

An analog PLL employs digital circuitry for calibration and characterization, precisely setting and maintaining the bandwidth of the PLL. A digital calibration circuit calibrates the value of a resistor or capacitor in the loop filter to yield a desired RC product. A digital control circuit reads time-to-digital converters (TDC) digitizing the length of the CU and CD pulses from the phase-frequency detector (PFD) to the charge pump (CP) during a frequency change. These pulse lengths are summed to yield a measured integral CP current. The control circuit determines an integral CP current that yields a desired bandwidth, regardless of the VCO tuning sensitivity, based on the calibrated RC product. The CP current is then adjusted by the ratio of determined integral CP current to the measured integral CP current. The digital circuits do not increase power consumption or adversely affect system operation.