AD6684 datasheet(39/107 Pages) AD

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Data Sheet

AD6684

Rev. 0 | Page 39 of 107

NCO FREQUENCY TUNING WORD (FTW) SELECTION

48-BIT NCO FTW = MIXING FREQUENCY/ADC SAMPLE RATE × 248

QUADRATURE ANALOG MIXER +

2 ADCs + QUADRATURE DIGITAL

MIXER + NCO

COMPLEX INPUT—SAMPLED AT

–

fS/32

BANDWIDTH OF

INTEREST

POSITIVE FTW VALUES

IMAGE DUE TO

ANALOG I/Q

MISMATCH

REAL

QUADRATURE MIXER

–

COMPLEX

ADC

SAMPLING

ADC

SAMPLING

90°

PHASE

48-BIT

NCO

90°

0°

48-BIT NCO FTW =

ROUND ((

fS/3)/fS × 248) =

+9.382513 (0x555555555555)

–

fS/2

–

fS/3

–

fS/4

–

fS/8

fS/16

fS/8

fS/4

fS/3

fS/2

–

fS/16

–

fS/32

Figure 76. DDC NCO Frequency Tuning Word Selection—Complex Inputs

DDC NCO + MIXER LOSS AND SFDR

When mixing a real input signal down to baseband, 6 dB of loss

is introduced in the signal due to filtering of the negative image.

An additional 0.05 dB of loss is introduced by the NCO. The

total loss of a real input signal mixed down to baseband is

6.05 dB. For this reason, it is recommended that the user

compensate for this loss by enabling the 6 dB of gain in the gain

stage of the DDC to recenter the dynamic range of the signal

within the full scale of the output bits.

When mixing a complex input signal down to baseband, the

maximum value that each I/Q sample can reach is 1.414 × full

scale after it passes through the complex mixer. To avoid over-

range of the I/Q samples and to keep the data bit widths aligned

with real mixing, 3.06 dB of loss is introduced in the mixer for

complex signals. An additional 0.05 dB of loss is introduced by

the NCO. The total loss of a complex input signal mixed down

to baseband is −3.11 dB.

The worst case spurious signal from the NCO is greater than

102 dBc SFDR for all output frequencies.

NUMERICALLY CONTROLLED OSCILLATOR

The AD6684 has a 48-bit NCO for each DDC that enables the

frequency translation process. The NCO allows the input

spectrum to be tuned to dc, where it can be effectively filtered

by the subsequent filter blocks to prevent aliasing. The NCO

can be set up by providing a frequency tuning word (FTW) and

a phase offset word (POW).

Setting Up the NCO FTW and POW

The NCO frequency value is given by the 48-bit twos

complement number entered in the NCO FTW. Frequencies

between −fS/2 and +fS/2 (fS/2 excluded) are represented using

the following frequency words:



0x8000 0000 0000 represents a frequency of −fS/2.



0x0000 0000 0000 represents dc (frequency is 0 Hz).



0x7FFF FFFF FFFF represents a frequency of +fS/2 − fS/248.

The NCO frequency tuning word can be calculated using the

following equation:

















FTW

NCO

mod

round

where:

NCO_FTW is a 48-bit twos complement number representing

the NCO FTW.

fC is the desired carrier frequency in Hz.

fS is the AD6684 sampling frequency (clock rate) in Hz.

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

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

round( ) is a rounding function. For example, round(3.6) = 4

and for negative numbers, round(–3.4) = −3.

Note that this equation applies to the aliasing of signals in the

digital domain (that is, aliasing introduced when digitizing

analog signals).

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