AD9694
Data Sheet
FREQUENCY TRANSLATION
GENERAL DESCRIPTION
Frequency translation is accomplished by using a 48-bit
complex NCO with a digital quadrature mixer. This stage
translates either a real or complex input signal from an IF to a
baseband complex digital output (carrier frequency = 0 Hz).
The frequency translation stage of each DDC can be controlled
individually and supports four different IF modes using Bits[5:4]
of the DDC control registers (Register 0x0310 and Register 0x0330)
in conjunction with the pair index register (Register 0x0009).
These IF modes are
Variable IF mode
0 Hz IF or zero IF (ZIF) mode
fS/4 Hz IF mode
Test mode
Variable IF Mode
NCO and mixers are enabled. NCO output frequency can be
used to digitally tune the IF frequency.
0 Hz IF (ZIF) Mode
The mixers are bypassed, and the NCO is disabled.
fS/4 Hz IF Mode
The mixers and the NCO are enabled in special downmixing by
fS/4 mode to save power.
Test Mode
Input samples are forced to 0.9599 to positive full scale. The
NCO is enabled. This test mode allows the NCOs to directly
drive the decimation filters.
Figure 74 and Figure 75 show examples of the frequency
translation stage for both real and complex inputs.
ADC + DIGITAL MIXER + NCO
REAL INPUT—SAMPLED AT fS
NCO FREQUENCY TUNING WORD (FTW) SELECTION
48-BIT NCO FTW = MIXING FREQUENCY/ADC SAMPLE RATE × 248
I
REAL
ADC
SAMPLING
REAL
AT fS
48-BIT
NCO
cos(ωt)
90°
0°
COMPLEX
–sin(ωt)
Q
BANDWIDTH OF
INTEREST IMAGE
BANDWIDTH OF
INTEREST
–fS/32 fS/32
–fS/2
–fS/3
–fS/4
–fS/8 –fS/16
DC
fS/16 fS/8
fS/4
fS/3
fS/2
–6dB LOSS DUE TO
NCO + MIXER
POSITIVE FTW VALUES
48-BIT NCO FTW =
ROUND ((fS/3)/fS × 248) =
+9.382513 (0x555555555555)
–fS/32 DC fS/32
48-BIT NCO FTW =
–9.3R8O25U1N3D(0(x(–FfFSF/3F)0/f0S0×00204080)0=000)
NEGATIVE FTW VALUES
–fS/32 DC fS/32
Figure 74. DDC NCO Frequency Tuning Word Selection—Real Inputs
Rev. 0 | Page 38 of 101