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AD9680

Data Sheet

Rev. C | Page 54 of 97

FREQUENCY TRANSLATION

FREQUENCY TRANSLATION GENERAL DESCRIPTION

Frequency translation is accomplished by using a 12-bit

complex NCO along with a digital quadrature mixer. The

frequency translation translates either a real or complex input

signal from an intermediate frequency (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 0x310, Register 0x330,

•

Variable IF mode

•

0 Hz 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

Mixers are bypassed and the NCO is disabled.

fS/4 Hz IF Mode

Mixers and NCO are enabled in special down mixing by fS/4

mode to save power.

Test Mode

Input samples are forced to 0.999 to positive full scale. NCO is

enabled. This test mode allows the NCOs to directly drive the

decimation filters.

Figure 148 and Figure 149 show examples of the frequency

translation stage for both real and complex inputs.

BANDWIDTH OF

INTEREST

BANDWIDTH OF

INTEREST IMAGE

NCO FREQUENCY TUNING WORD (FTW) SELECTION

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

ADC + DIGITAL MIXER + NCO

REAL 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

–

fS/32

cos(ωt)

0°

90°

ADC

SAMPLING

REAL

–sin(ωt)

12-BIT

NCO

POSITIVE FTW VALUES

NEGATIVE FTW VALUES

COMPLEX

–6dB LOSS DUE TO

NCO + MIXER

12-BIT NCO FTW =

ROUND ((

fS/3)/fS × 4096) = +1365 (0x555)

12-BIT NCO FTW =

ROUND ((

fS/3)/fS × 4096) = –1365 (0xAAB)

Figure 148. DDC NCO Frequency Tuning Word Selection—Real Inputs

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