Direct QAM modulator with digital feedback control and complex pre-equalization for phase and amplitude distortion

Abstract

A direct quadrature amplitude modulation (QAM) modulator provides amplitude and phase pre-equalization to reduce complexity and cost. In-phase (I) and quadrature-phase (Q) QAM signal components are provided from an analog modulator. A radio-frequency (RF) driver provides a RF signal with the QAM components. A monitoring device monitors phase and amplitude errors in the RF signal, and provides a feedback signal to a digital complex phase and amplitude equalizer embedded in a digital Nyquist filter. The equalizer uses the feedback signal to provide a pre-equalizing signal to a digital-to-analog converter, which provides a corresponding analog equalizing signal. This signal is, in turn, low-pass filtered and fed back to the analog I/Q modulator to equalize the phase and amplitude of the I and Q components there. The invention is particularly suitable for generating 64/256-QAM signals in the 50 MHz-880 MHz range without an Intermediate Frequency (IF) stage and double-conversion, and may be used, e.g., for communicating digital television data via a cable network.

Dependencies are inferred from the claim text.