AD6649EBZ datasheet(25/40 Pages) AD

25 / 40 page

AD6649

Rev. 0 | Page 25 of 40

–10

–20

–30

–40

–50

–60

–70

–80

–90

–100

–110

–120

30.72

61.44

92.16

122.88

FILTER RESPONSE (MHz)

Figure 43. High Performance FIR Filter Pass-Band Response at 245.76 MSPS

(Tunable-Frequency NCO, 100 MHz FIR Filter)

–0.1

–0.2

–0.3

–0.4

–0.5

–0.6

–0.7

–0.8

–0.9

–1.0

15.36

30.72

46.08

61.44

FILTER RESPONSE (MHz)

Figure 44. High Performance FIR Filter Pass-Band Response at 245.76 MSPS

(Tunable-Frequency NCO, 100 MHz FIR Filter)

OUTPUT NCO

The output of the 32-bit fine-tuning NCO is complex and

typically centered in frequency around dc. This complex output

is carried through the stages of either the 95 MHz or 100 MHz FIR

filter to provide proper antialiasing filtering. The final NCO

provides a means to move this complex output signal away from

dc so that a real output can be provided from the AD6649. The

output NCO translates the output from dc to a frequency equal

to the output frequency divided by 4 (fS/4). This provides the user

with an output signal centered at fS/4 in frequency.

The AD6649 output NCOs within a single part or across

multiple parts can be synchronized using the external SYNC

input. Bit 7 and Bit 6 of Register 0x58 allow the output NCO to

be resynchronized on every SYNC signal or only on the first

SYNC signal after the register is written.

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