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AD6679

Data Sheet

Rev. B | Page 46 of 81

FREQUENCY TRANSLATION

GENERAL DESCRIPTION

Frequency translation is accomplished by using a 12-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 0x310, Register 0x330,



Variable IF mode



0 Hz IF, or zero IF (ZIF), mode



fS/4 Hz IF mode



Test mode

Variable IF Mode

The NCO and the mixers are enabled. The 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 a special downmixing

by fS/4 mode to save power.

Test Mode

The input samples are forced to 0.999 to positive full scale. The

NCO is enabled. This test mode allows the NCOs to drive the

decimation filters directly.

Figure 65 and Figure 66 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(wt)

0°

90°

ADC

SAMPLING

REAL

–sin(wt)

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 65. DDC NCO Frequency Tuning Word Selection—Real Inputs

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