AD9854

Rev. E | Page 13 of 52

Figure 10 to Figure 15 show the trade-off in elevated noise floor, increased phase noise (PN), and discrete spurious energy when the internal

REFCLK multiplier circuit is engaged. Plots with wide (1 MHz) and narrow (50 kHz) spans are shown. Compare the noise floor of

Figure 11 and Figure 12 with that of Figure 14 and Figure 15. The improvement seen in Figure 11 and Figure 12 is a direct result of

sampling the fundamental at a higher rate. Sampling at a higher rate spreads the quantization noise of the DAC over a wider bandwidth,

which effectively lowers the noise floor.

0

CENTER 39.1MHz

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–50

–60

–70

–80

–90

–100

100kHz/

SPAN 1MHz

Figure 10. Narrow-Band SFDR, 39.1 MHz, 1 MHz BW,

300 MHz REFCLK with REFCLK Multiplier Bypassed

0

CENTER 39.1MHz

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–40

–50

–60

–70

–80

–90

–100

5kHz/

SPAN 50kHz

Figure 11. Narrow-Band SFDR, 39.1 MHz, 50 kHz BW,

300 MHz REFCLK with REFCLK Multiplier Bypassed

0

CENTER 39.1MHz

–10

–20

–30

–40

–50

–60

–70

–80

–90

–100

5kHz/

SPAN 50kHz

Figure 12. Narrow-Band SFDR, 39.1 MHz, 50 kHz BW,

100 MHz REFCLK with REFCLK Multiplier Bypassed

0

CENTER 39.1MHz

–10

–20

–30

–40

–50

–60

–70

–80

–90

–100

100kHz/

SPAN 1MHz

Figure 13. Narrow-Band SFDR, 39.1 MHz, 1 MHz BW,

30 MHz REFCLK with REFCLK Multiplier = 10×

0

CENTER 39.1MHz

–10

–20

–30

–40

–50

–60

–70

–80

–90

–100

5kHz/

SPAN 50kHz

Figure 14. Narrow-Band SFDR, 39.1 MHz, 50 kHz BW,

30 MHz REFCLK with REFCLK Multiplier = 10×

0

CENTER 39.1MHz

–10

–20

–30

–40

–50

–60

–70

–80

–90

–100

5kHz/

SPAN 50kHz

Figure 15. Narrow-Band SFDR, 39.1 MHz, 50 kHz BW,

10 MHz REFCLK with REFCLK Multiplier = 10×