Methods and apparatus for controlling a controlled oscillator using a phase-locked loop (PLL) or frequency-locked loop (FLL) having a digital
loop filter with programmable filter parameters. An exemplary PLL (or FLL) includes a digital
loop filter having one or more of the programmable filter parameters, which are changed by increments during operation in order to minimize disturbances (e.g.,
settling transients) as the
loop bandwidth of the PLL is varied from a narrow
loop bandwidth to a wide
loop bandwidth, or vice versa. By changing the
loop filter parameters in increments the loop bandwidth can be varied with substantially no perturbation. The end result is a much faster frequency
switching time, improved
settling dynamics, and predictable and stable loop operating performance. According to another aspect of the invention, one or more of the programmable filter parameters are changed in order to oppose a change in tuning sensitivity of the controlled oscillator (e.g., in order to maintain a constant loop bandwidth). By holding the loop bandwidth constant,
switching time is maintained substantially constant under all conditions. This allows design and production margins to be reduced in a frequency agile
system, and also relaxes the tuning sensitivity
linearity requirements of the controlled oscillator.