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Data Sheet

AD9680

Rev. C | Page 55 of 97

NCO FREQUENCY TUNING WORD (FTW) SELECTION

12-BIT NCO FTW = MIXING FREQUENCY/ADC SAMPLE RATE × 4096

QUADRATURE ANALOG MIXER +

2 ADCs + QUADRATURE DIGITAL

MIXER + NCO

COMPLEX INPUT—SAMPLED AT

–

fS/2

–

fS/3

–

fS/4

–

fS/8

fS/16

fS/8

fS/4

fS/3

fS/2

–

fS/16

–

fS/32

–

fS/32

12-BIT NCO FTW =

ROUND ((

fS/3)/fS × 4096) = +1365 (0x555)

BANDWIDTH OF

INTEREST

POSITIVE FTW VALUES

IMAGE DUE TO

ANALOG I/Q

MISMATCH

REAL

QUADRATURE MIXER

–

COMPLEX

ADC

SAMPLING

ADC

SAMPLING

90°

PHASE

12-BIT

NCO

90°

0°

Figure 149. DDC NCO Frequency Tuning Word Selection—Complex Inputs

DDC NCO PLUS 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 additional 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 each I/Q sample can reach is 1.414 × full scale

after it passes through the complex mixer. To avoid overrange

of the I/Q samples and to keep the data bitwidths aligned with

real mixing, 3.06 dB of loss (0.707 × full scale) 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 AD9680 has a 12-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 12-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:

•

0x800 represents a frequency of –fS/2.

•

0x000 represents dc (frequency is 0 Hz).

•

0x7FF represents a frequency of +fS/2 – fS/212.

The NCO frequency tuning word can be calculated using the

following equation:













Mod

round

FTW

NCO

)

(

where:

NCO_FTW is a 12-bit twos complement number representing

the NCO FTW.

fS is the AD9680 sampling frequency (clock rate) in Hz.

fC is the desired carrier frequency 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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