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Optimising Arduino, AD9851 DDS signal generator

09 Mar 2016  | Steve Sandler

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The filter was assembled on a PCB board, shown in figure 11. The filter was connected differentially across the two AD9851 outputs and connected to a 50 Ω oscilloscope using a wide bandwidth DC block. The DC block eliminates the 50 Ω DC loading on the AD9851.


Figure 11: The reduced component count Chebyshev filter was constructed on a PCB and connected to a 50 Ω oscilloscope input using the Picotest P2130A (500Hz—8Ghz) DC block.


The 40MHz time domain response shown in figure 12 is clean, even at high frequency output owing to the filter trap. The amplitude roll-off is very sharp above 47MHz.


Figure 12: The 40MHz time domain response shows a clean output and not quite −3dB from the low frequency output. The amplitude roll-off is very sharp about 47MHz.


Figure 13 shows the spectral content for a 45MHz output, where the images would be most severe. The first signal images are below −60 dBc. The residual 125MHz clock and the clock harmonics are the largest spurs. The spurs are all below about −47 dBc up to 500MHz.


Figure 13: The first signal images are below −60 dBc. The residual 125MHz clock and the clock harmonics are the largest spurs. The spurs are all below about −47 dBc up to 500MHz.


Conclusion
A Keysight ADS harmonic balance simulation model was used to improve the performance of the AD9851 DDS waveform generator while also reducing the output filter component count. A Chebyshev low pass filter, incorporating a 65MHz trap was designed for a 50 Ω output, taken differentially across the two AD9851 outputs. The filter was designed to provide optimum gain flatness while improving the first signal image by nearly 40 dB at 45MHz. The output measured −3 dB at approximately 46MHz, very close to the 50MHz design goal, with the error likely being due to the chip inductor tolerances. The spectral content could likely be further improved by reducing the signal level via Rset. The 50 Ω output simplifies wideband connections using coax cable and is also compatible with additional RF amplifiers and filters.

The DDS waveform generator can also be used in narrow band applications using a bandpass filter. The output signal can also be obtained from signal images rather than the fundamental signal, allowing operation well above the 125MHz clock frequency. Perhaps we'll look at these applications in a future article.


About the author
Steve Sandler is the founder and former CEO of Analytical Engineering Inc., the predecessor of AEi Systems. He has over 30 years experience in the design and analysis of power conversion equipment for military and space applications. Mr. Sandler is also the CEO of Picotest.com, a company that distributes test equipment including the Signal Injector product line designed for testing linear and switching power supplies, a Test & Measurement World "Best in Test" Finalist.


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