Phase-locked loop filter with coarse and fine tuning

摘要

A loop filter for a phase locked loop (PLL) having fast tuning capability while limiting phase noise. The filter includes a fine tune input port to receive a fine tune signal from the phase detector and a coarse tune input port to receive a coarse tune signal from the coarse tuner. The external coarse tuner provides the majority of the voltage slew on the loop filter while a fine tune control, thus reducing tune time. In one embodiment, the loop filter includes a voltage divider to limit the effective tuning sensitivity and thus control noise induced on a voltage-controlled oscillator from the loop filter. An elliptical filter may be employed to attenuate fractional spurs within the filter output signal.

主权项

1. A circuit comprising:
a fine tune input port to receive a fine tune signal; a coarse tune input port to receive a coarse tune signal; an output port; a first summing element coupled to the fine tune and coarse tune input ports to sum the fine tune and coarse tune signals and to output a summed tune signal; a voltage divider coupled to the first summing element to receive the summed tune signal and to output a voltage divided signal; and a smoothing filter coupled to the voltage divider to smooth the voltage divided signal and output a smoothed signal to the output port.