Direct digital synthesizer modeling at frequency images

An attempt to implement AD9912 DDS at first and second image frequency revealed a noticeable discrepancy of calculated phase noise with the measurement results, as particular the output phase noise at 1080 MHz was less then at 840 MHz with clock frequency 1 GHz. To mitigate this mismatch a new reconstruction pulse model was developed in assumption that D/A converter of DDS consists of two interleaved converters active for half a clock time. The results of calculations of frequency response both with phase and amplitude noise spectral density with proposed model are given. The experiment demonstrates a good match with model for phase and amplitude noise up to fifth frequency image. Measurements made with DDS AD9910 show similar results. At the same time, AD9914 has a normal frequency response, and its phase noise changes as f 2 up to 3,5 GHz with clock frequency 1 GHz.