AD6679 datasheet(48/82 Pages) AD

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Data Sheet

AD6679

Rev. B | Page 47 of 81

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 66. 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.

The NCO introduces an additional 0.05 dB of loss. The total

loss of a real input signal mixed down to baseband is 6.05 dB.

For this reason, it is recommended to 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 each I/Q sample can reach is 1.414 × full scale

after it passes through the complex mixer. To avoid an

overrange 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. The NCO introduces an additional

0.05 dB of loss. 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 AD6679 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.

Calculate the NCO frequency tuning word using the following

equation:

















FTW

NCO

mod

round

where:

NCO_FTW is a 12-bit, twos complement number representing

the NCO FTW.

fC is the desired carrier frequency in Hz.

fS is the AD6679 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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