Experiment with AD9851 DDS OH2NLT 27.08.2004-08-27
Original AD9850 synthesizer has 125MHz-reference oscillator what sets Nyquist frequency at 62,5MHz. Practical maximum output frequency from AD9850 is however about 50MHz. For our four phase multiplexer-mixer we need f-in*4 LO frequency. This limits us to maximum 12,5MHz operation.
AD9850 sister part AD9851 has reference 180MHz clock what gives us 72 to 74MHz output what enables us 18MHz band. AD9851 requires different comparator low-pass filter and has one control bit different from AD9850. Also some constants and scaling need to be changed. AD9850 tuning step is 0.0291Hz/bit and AD9851 tuning step is 0.0419Hz/bit. AD9850 180MHz reference frequency is multiplied from 30MHz external oscillator.
You can find test software for AD9851 here. Functionality is same as in v1.02 software for AD9850. AD9851 modified DDS schematics. 70MHz low-pass filter simulation. New low-pass filter patched to DDS board DDS analog output signal at 60MHz (center).Horizontal scale is 10MHz/div Right bottom corner spikes are local BC stations coming through. AD9851 datasheet