Programmable loop filter for carrier recovery in a radio receiver

Abstract

A digital loop filter in the carrier-recovery loop of a digital communications receiver. The recovery loop is a PLL that keeps the receiver oscillator locked to the carrier wave, and the loop filter provides control over the PLL's frequency response by conditioning an error signal that is fed back to the receiver oscillator. In the present invention, the error signal is a digital signal, and the loop filter is implemented in digital hardware. With this implementation the characteristics of the loop filter are determined by logic design rather than by physical features of analog components, thereby giving this filter a more precise function than one with analog integrators. This implementation is also immune to the low tolerances typical of the manufacturing process for analog devices (especially on monolithic circuits), and is more easily adjusted than its analog counterparts. Two gain coefficients characterize the loop filter in the present invention. These gain coefficients are chosen to be powers of two, simplifying the process of multiplying them with the digital error signal. The gain coefficients are read from a memory, making the loop filter easily programmable. By changing the gain coefficients during operation of the receiver, the carrier-recovery loop can be placed in one of the several operating modes, including acquisition, tracking, and hold. The receiver can be configured with the appropriate values of the gain coefficients for each operating mode during the initial assembly and during subsequent reconfigurations.