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Data Sheet

AD9684

FREQUENCY TRANSLATION

GENERAL DESCRIPTION

Frequency translation is accomplished using a 12-bit complex

NCO with a digital quadrature mixer. The frequency translation

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 × full scale to positive full

scale. The NCO is enabled. This test mode allows the NCOs to

drive the decimation filters directly.

Figure 60 and Figure 61 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 60. DDC NCO Frequency Tuning Word Selection—Real Inputs

Rev. 0 | Page 37 of 64

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