AD9689-2000EBZ datasheet(49/134 Pages) AD

49 / 134 page

Data Sheet

AD9689

Rev. A | Page 49 of 134

DDC NCO Description

Each DDC contains one NCO. Each NCO enables the

frequency translation process by creating a complex exponential

frequency (e-jωct), which can be mixed with the input spectrum

to translate the desired frequency band of interest to dc, where

it can be filtered by the subsequent low-pass filter blocks to

prevent aliasing.

When placed in variable IF mode, the NCO supports two

additional modes.

DDC NCO Programmable Modulus Mode

DDC NCO programmable modulus mode supports >48-bit

frequency tuning accuracy for applications that require exact

rational (M/N) frequency synthesis at a single carrier frequency.

In this mode, the NCO is set up by providing the following:

•

48-bit frequency tuning word (FTW)

•

48-bit Modulus A word (MAW)

•

48-bit Modulus B word (MBW)

•

48-bit phase offset word (POW)

DDC NCO Coherent Mode

DDC NCO coherent mode allows an infinite number of frequency

hops where the phase is referenced to a single synchronization

event at Time 0. This mode is useful when phase coherency

must be maintained when switching between different frequency

bands. In this mode, the user can switch to any tuning frequency

without the need to reset the NCO. Although only one FTW is

required, the NCO contains 16 shadow registers for fast

switching applications. Selection of the shadow registers is

controlled by the CMOS GPIO pins or through the register map

of the SPI. In this mode, the NCO can be set up by providing

the following:

•

Up to sixteen 48-bit FTWs.

•

Up to sixteen 48-bit POWs.

•

The 48-bit MAW must be set to zero in coherent mode.

Figure 113 shows a block diagram of one NCO and its connection

to the rest of the design. The coherent phase accumulator block

contains the logic that allows an infinite number of frequency

hops. The gray lines in Figure 113 represent SPI control lines.

NCO

COS/SIN

GENERATOR

NCO CHANNEL

SELECTION

SYSREF

DIGITAL

QUADRATURE

MIXER

FTW = FREQUENCY TUNING WORD

POW = PHASE OFFSET WORD

MAW = MODULUS A WORD (NUMERATOR)

MBW = MODULUS B WORD (DENOMINATOR)

COHERENT

PHASE

ACCUMULATOR

BLOCK

48-BIT

MAW/MBW

MODULUS

ERROR

NCO

CHANNEL

SELECTION

CIRCUITS

I/O

CROSSBAR

MUX

SYNCHRONIZATION

CONTROL CIRCUITS

DECIMATION

FILTERS

MAW/MBW

FTW/POW

WRITE INDEX

48-BIT

FTW/POW

48-BIT

FTW/POW

48-BIT

FTW/POW

MAP

Figure 113. NCO + Mixer Block Diagram

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