Direct digital frequency synthesizer using sigma-delta techniques

Abstract

A direct digital synthesizer (DDS) for generating a waveform generates a sequence of n-bit phase signals representing phase of the waveform, wherein n is an integer greater than zero. Each n-bit phase signal comprises a phase estimate signal and a phase error signal. The phase estimate signal comprises a most-significant m bits of the n-bit quantity (0 <m<n). The phase-error signal comprises a least-significant n-m bits of the n-bit quantity. The DDS further has a sigma-delta modulator for generating a compensation signal from the phase error signals. The phase estimate signal is added to the compensation signal to produce a compensated phase signal, which may be used to address a look-up table having waveform samples stored in correspondence with look-up table addresses. A digital to analog converter may be coupled to receive waveform samples from the look-up table in order to generate a corresponding analog waveform signal. In accordance with another aspect of the invention, an improved modulator includes the improved DDS as described above, for generating a stream of carrier signal samples; and a complex multiplier/accumulator (CMAC) for multiplying a stream of the carrier signal samples by a stream of complex data values that represent a baseband signal. The output of the CMAC is a stream of modulated carrier signal samples that may be supplied to a digital to analog converter which generates a corresponding analog modulated carrier signal.