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Optimizing Arduino and the AD9851 DDS signal generator

-February 10, 2016

The output filter

The Analog Devices evaluation board and the Ebay board both use the same 7th order elliptical output filter design. The filter is published in the datasheet and is shown in Figure 6.


Figure 6 70 MHz elliptical filter used in Ebay board and shown in the Analog Devices datasheet.

While the elliptical filter is sharp, the 70 MHz filter cutoff is too high for the 125 MHz clock, resulting in poor image rejection at 50 MHz, as seen in Figure 7.


Figure 7 ADS simulation of the 70 MHz filter shown in Figure 6.

A Chebyshev low pass filter with a 65 MHz trap is shown in Figure 8. The filter is designed to mate directly with a 50 Ω load, though with a reduced amplitude. The sharp trap significantly reduces the images at higher output frequencies, while also using fewer components.


Figure 8 Reduced component count Chebyshev low pass filter incorporating a 65 MHz trap.

The improved filter provides nearly 50 dB better image rejection at 50 MHz. The amplitude can be restored to the prior level using the Rset amplitude adjustment resistor. A comparison between the 70 MHz elliptical filter and the improved Chebyshev low pass filter incorporating a trap is shown in Figure 9.


Figure 9 Comparison between the 70 MHz elliptical filter and the improved Chebyshev low pass filter. The Chebyshev filter is designed to be −3dB at approximately 50 MHz.

The signal amplitude is reduced a bit using the Chebyshev filter due to the 50 Ω output loading. While the amplitude can be adjusted by reducing the value of the Rset resistor doing so will significantly increase both the odd and even low order harmonics of the output waveform. The spectral content of the filtered output is shown for both filters in Figure 10.


Figure 10 With a 45 MHz output signal the elliptical filter indicates an image only −24 dBc for the elliptical filter while the reduced component low pass Chebyshev filter results in nearly −65 dBc improving the performance by 40 dB while also improving the amplitude flatness and reducing component count.

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 40 MHz 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 47 MHz.  


Figure 12 The 40 MHz 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 45 MHz output, where the images would be most severe. The first signal images are below −60 dBc. The residual 125 MHz clock and the clock harmonics are the largest spurs. The spurs are all below about −47 dBc up to 500 MHz.


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

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 65 MHz 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 45 MHz. The output measured −3 dB at approximately 46 MHz, very close to the 50 MHz 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 125 MHz clock frequency. Perhaps we’ll look at these applications in a future article.

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