Data Sheet
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
REAL
I
ADC
SAMPLING
AT fS
90°
PHASE
Q
ADC
SAMPLING
AT fS
IMAGE DUE TO
ANALOG I/Q
MISMATCH
QUADRATURE MIXER
I
I
+I
I Q–
48-BIT
NCO
Q
90°
0°
Q
COMPLEX
Q
QI I +
Q
+
BANDWIDTH OF
INTEREST
–fS/2
–fS/3
–fS/4
–fS/32 fS/32
–fS/8 –fS/16
DC
fS/16 fS/8
fS/4
fS/3
AD9694
fS/2
POSITIVE FTW VALUES
48-BIT NCO FTW =
ROUND ((fS/3)/fS × 248) =
+9.382513 (0x555555555555)
–fS/32 fS/32
DC
Figure 75. DDC NCO Frequency Tuning Word Selection—Complex Inputs
DDC NCO AND 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
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. 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 AD9694 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 32-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:
NCO _ FTW
round
2 48
mod fC , fS
fS
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 AD9694 sampling frequency (clock rate) in Hz.
round( ) is a rounding function. For example, round(3.6) = 4
and for negative numbers, round(–3.4) = −3.
mod( ) is a remainder function. For example, mod(110,100) =
10 and for negative numbers, mod(–32,10) = −2.
Note that this equation applies to the aliasing of signals in the
digital domain (that is, aliasing introduced when digitizing
analog signals).
Rev. 0 | Page 39 of 101