AD6688 datasheet(48/138 Pages) AD

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AD6688

Data Sheet

Rev. 0 | Page 48 of 138

NCO FTW/POW/MAW/MAB Description

The NCO frequency value is determined by the following settings:

•

A 48-bit twos complement number entered in the FTW.

•

A 48-bit unsigned number entered in the MAW.

•

A 48-bit unsigned number entered in the MBW.

Frequencies between −fS/2 and +fS/2 (fS/2 excluded) are

represented using the following values:

•

FTW = 0x8000_0000_0000 and MAW = 0x0000_0000_0000

represents a frequency of –fS/2.

•

FTW = 0x0000_0000_0000 and MAW = 0x0000_0000_0000

represents dc (frequency is 0 Hz).

•

FTW = 0x7FFF_FFFF_FFFF and MAW = 0x0000_0000_0000

represents a frequency of +fS/2.

NCO FTW/POW/MAW/MAB Programmable Modulus

Mode

For programmable modulus mode, the MAW must be set to a

nonzero value (not equal to 0x0000_0000_0000). This mode is

only needed when frequency accuracy of >48 bits is required. One

example of a rational frequency synthesis requirement that requires

>48 bits of accuracy is a carrier frequency of 1/3 the sample

rate. When frequency accuracy of ≤48 bits is required, coherent

mode must be used (see the NCO FTW/POW/MAW/MAB

Coherent Mode section).

In programmable modulus mode, the FTW, MAW, and MBW

must satisfy the following four equations (for a detailed

description of the programmable modulus feature, see the DDS

architecture described in the AN-953 Application Note):

)

mod(

MBW

MAW

FTW

(1)

)

mod(

floor( 48

FTW =

(2)

MAW = mod(248 × M,N)

(3)

MBW = N

(4)

where:

fC is the desired carrier frequency.

fS is the ADC sampling frequency.

M is the integer representing the rational numerator of the

frequency ratio.

N is the integer representing the rational denominator of the

frequency ratio.

FTW is the 48-bit twos complement number representing the

NCO FTW.

MAW is the 48-bit unsigned number representing the NCO

MAW (must be <247).

MBW is the 48-bit unsigned number representing the NCO MBW.

mod(x) is a remainder function. For example mod(110,100) =

10 and for negative numbers, mod(–32,10)= –2.

floor(x) is defined as the largest integer less than or equal to x.

For example, floor(3.6) = 3.

Note that Equation 1 to Equation 4 apply to the aliasing of

signals in the digital domain (that is, aliasing introduced when

digitizing analog signals).

M and N are integers reduced to their lowest terms. MAW and

MBW are integers reduced to their lowest terms. When MAW is

set to zero, the programmable modulus logic is automatically

disabled.

For example, if the ADC sampling frequency (fS) is 3000 MSPS

and the carrier frequency (fC) is 1001.5 MHz, then,

6000

2003

3000

)

3000

1001

mod(

0_FB38

0x5576_19F

)

3000

)

3000

1001

mod(

floor( 48

FTW

MAW = mod(248 × 2003, 6000) = 0x0000_0000_0F80

MBW = 0x0000_0000_1770

The actual carrier frequency can be calculated based on the

following equation:

ACTUAL

MBW

MAW

FTW

For the previous example, the actual carrier frequency (fC_ACTUAL)

MHz

1001

0_1770

0x0000_000

0_0F80

0x0000_000

0_FB38

0x5576_19F

ACTUAL

A 48-bit POW is available for each NCO to create a known

phase relationship between multiple chips or individual DDC

channels inside the chip.

While in programmable modulus mode, the FTW and POW

registers can be updated at any time while still maintaining

deterministic phase results in the NCO. However, the following

procedure must be followed to update the MAW and/or MBW

registers to ensure proper operation of the NCO:

1. Write to the MAW and MBW registers for all the DDCs.

2. Synchronize the NCOs either through the DDC soft reset

bit accessible through the SPI or through the assertion of

the SYSREF± pin (see the Memory Map section).

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