AD9854

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REV. 0

Figures 14 and 15 show the effects of utilizing “sweet spots” in the tuning range of a DDS. Figure 14 represents a tuning word that

accentuates the aberrations associated with truncation in the DDS algorithm. Figure 16 is essentially the same output frequency (a

few tuning codes over), but it displays much fewer spurs on the output due to the selection of a tuning “sweet spot.” Consideration

should be given to all DDS applications to exploit the benefit of sweet spot tuning.

0

CENTER 112.499MHz

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SPAN 500kHz

Figure 14. The Opposite of a “Sweet Spot.” 112.469 MHz

with multiple high energy spurs close around the

fundamental.

0

CENTER 112.469MHz

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SPAN 500kHz

Figure 15. A slight change in tuning word yields

dramatically better results. 112.499 MHz with all

spurs shifted out-of-band.

0

CENTER 39.1MHz

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SPAN 50kHz

Figure 16. Narrowband SFDR, 39.1 MHz, 50 kHz BW,

200 MHz EXTCLK with REFCLK Multiplier Bypassed

Figures 16 and 17 show the narrowband performance of the AD9854 when operating with a 20 MHz reference clock and the

REFCLK Multiplier enabled at 10

× vs. a 200 MHz external reference clock.

0

CENTER 39.1MHz

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Figure 17. Narrowband SFDR, 39.1 MHz, 50 kHz BW,

10 MHz EXTCLK with REFCLK Multiplier = 10

×