Frequency synthesizers adaptive loop filter with compensation for transients

摘要

An improved adaptive loop filter for modulatable frequency synthesizers containing digital phase comparators is described. The unique adaptive loop filter provides a wide bandwidth during a signal acquisition mode and a narrow bandwidth during a signal tracking mode. The adaptive loop filter includes a current limiter coupled to the error signal from the digital phase comparator for generating a current-limited output signal; a filter coupled by a resistor to the current limiter output signal and including a series-coupled resistor and capacitor for filtering the current limiter output signal to provide a steering line voltage; an amplifier coupled to the junction between the filter resistor and capacitor for generating a feedback signal; a resistor for coupling the feedback signal to the current limiter output signal for substantially reducing the duration of voltage transients; and a reference filter for coupling the steering line voltage to the steering input of a voltage-controlled oscillator. The wide acquisition loop bandwidth and the narrow tracking loop bandwidth are provided by switchably coupling different resistors to the filter capacitor. The phase shift produced by the reference filter is also changed depending on whether the acquisition or steering mode is active.