AD9213 datasheet(39/110 Pages) AD

39 / 110 page

Data Sheet

AD9213

Rev. A | Page 39 of 110

NCO FTW, POW, MAW, and MAB Description

The NCO frequency value is determined by the following

settings:



48-bit, twos complement number entered in the FTW



48-bit, unsigned number entered in the MAW



48-bit, unsigned number entered in the MBW

Frequencies between −fS/2 and +fS/2 (+fS/2) 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, and MAB Programmable

Modulus Mode Example Calculation

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,

and 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 AN-953

Application Note):





mod(

)

MAW

FTW

MBW

(1)



48 mod(

)

floor(2

)

FTW

(2)

MAW = mod(248 × M, N)

(3)

MBW = N

(4)

where:

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

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

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.

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

For example, floor(3.6) = 3.

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 fS is 10 GSPS and fC is 3 1/3 GHz,

mod 3 , 10





 



mod 3 ,10

floor 2

FTW















= 0x5555_5555_5555

MAW = mod(248 × 1, 3) = 0x0000_0000_0001

MBW = 0x0000_0000_0003

The actual carrier frequency (fC_ACTUAL) can be calculated based

on the following equation:

ACTUAL

MBW

MAW

FTW







For the previous example, fC_ACTUAL is,

fC_ACTUAL =

0x0000 _ 0000 _ 0001

0x5555_5555_5555

0x0000 _ 0000 _ 0003

10 GHz = 3 GHz





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.

In programmable modulus mode, the deterministic phase in the

NCO is not maintained when the FTW and POW are updated.

NCO FTW, POW, MAW, and MAB Coherent Mode

Example Calculation

For coherent mode, the NCO MAW must be set to zero

(0x0000_0000_0000). In this mode, the NCO FTW can be

calculated by the following equation:



48 mod(

, )

round(2

)

FTW

(5)

Where round(x) is the rounding function which rounds x to the

nearest integer. For example round(3.6) = 4.

Equation 5 applies to the aliasing of signals in the digital

domain (that is, aliasing introduced when digitizing analog

signals). The MAW must be set to zero to use coherent mode.

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