Method and apparatus providing an amplitude independent automatic frequency control circuit

Abstract

An automatic frequency control (AFC) loop circuit ( A, B) includes a mixer ( ) for mixing a local oscillator signal ( ) with a received signal to generate a result signal; a frequency detector ( ) coupled to the result signal and outputting a loop error signal; a combiner circuit ( ) for combining error signals output from a plurality of fingers ( ) of a rake receiver ( ) with the loop error signal output from the frequency detector to generate a combined loop error signal; a sign detector ( ) having an input coupled to an output of said combiner circuit for outputting a sign detector output signal for indicating a sign of the combined loop error signal; control circuitry ( ) for controlling a frequency of the local oscillator signal in accordance with the sign detector output signal and a loop filter ( ) partitioned into a first loop filter ( A) coupled between the output of the frequency detector and an input to the sign detector, and a second loop filter ( B) coupled between an output of the sign detector and an input to the control circuitry. In one embodiment the first loop filter is coupled between the output of the frequency detector and an input to the combiner circuit, while in another embodiment the first loop filter is coupled between the output of the combiner circuit and the input to the sign detector. The loop bandwidth is controlled with the loop filter, and the total required amount of loop filtering is split between the first loop filter and the second loop filter. In the preferred embodiment the combiner circuit comprises a Maximal Ratio Combining (MRC) circuit.