Spur Correlation in an Array of Direct Digital Synthesizers

Many applications, including communications, test and measurement, and radar, require the generation of signals with a high degree of spectral purity. One method for producing tunable, low-noise signals is to combine the outputs of multiple direct digital synthesizers (DDSs) arranged in a parallel configuration. In such an approach, if all noise is uncorrelated across channels, the noise power will decrease relative to the combined signal power, increasing the signal-to-noise ratio. However, if the noise or spurious components are correlated, the gains achieved by parallelization will be limited. This work examines the potential correlation of spurious components in an array of DDSs, with a focus on phase truncation spurs, quantization noise, and spurs from quantizer nonlinearities. We measured the levels of correlation among DDS channels on a custom 14-channel DDS testbed. Our study shows that the phase truncation spurs are uncorrelated, at least in our system. We believe this decorrelation is due to the existence of a mechanism in our DDS array that is unaccounted for in our current DDS model. This mechanism, likely due to some timing element in the FPGA, randomizes the relative phases of the truncation spurs from channel to channel each time the DDS array is powered on. This randomness decorrelates the phase truncation spurs, providing the potential for spur-free dynamic range improvement from a DDS array. Our measurements also show that the quantization noise of each DDS channel is uncorrelated for 3-bit or higher digital-to-analog converters (DAC). This suggests that for an N-channel array of DDSs, a near N gain in signal-to-quantization noise is possible. This gain will be most apparent for low-bit DACs in which quantization noise is notably higher than the thermal noise contribution. Lastly, our measurements of quantizer nonlinearity spurs demonstrate that the second and third harmonics are highly correlated across channels for all frequencies tested, suggesting that there is no benefit to using an array of DDSs for implementations in which in-band quantizer nonlinearities dominate. As a result, alternate methods of harmonic spur management must be employed to mitigate these errors. 42 nd Annual Precise Time and Time Interval (PTTI) Meeting