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Optimising Arduino, AD9851 DDS signal generator

09 Mar 2016  | Steve Sandler

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The output frequency is set to 50MHz and the unfiltered output spectrum is shown in figure 4. Now we can see the first and second images appear at 75MHz and 175MHz respectively, as expected. The first image amplitude is only 2.5 dB lower than the fundamental and also less than an octave from the fundamental, making it difficult to filter.


Figure 4: The DDS unfiltered output spectrum with a 50MHz output frequency.


Further increasing the frequency to 60MHz the unfiltered output spectrum is shown in figure 5. Now we can see the first and second images appear at 65MHz and 185MHz respectively, again as expected. Now the first image amplitude is only 0.5 dB lower than the fundamental and very close to the fundamental, making it even more difficult to filter.


Figure 5: The DDS unfiltered output spectrum with a 60MHz output frequency.


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


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


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


A Chebyshev low pass filter with a 65MHz 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 65MHz trap.


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


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


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 45MHz 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.



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