AD9689-2000EBZ datasheet(47/134 Pages) AD

47 / 134 page

Data Sheet

AD9689

Rev. A | Page 47 of 134

DDC FREQUENCY TRANSLATION

DDC 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 DDCx control registers (Register 0x0310, Register

0x0330, Register 0x0350, and Register 0x0370). 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

In variable IF mode, the NCO and mixers are enabled. NCO

output frequency can be used to digitally tune the IF frequency.

0 Hz IF (ZIF) Mode

In ZIF mode, the mixers are bypassed, and the NCO is disabled.

fS/4 Hz IF Mode

In fS/4 Hz IF mode, the mixers and the NCO are enabled in

downmixing by fS/4 mode to save power.

Test Mode

In test mode, input samples are forced to 0.999 to positive full

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

directly drive the decimation filters.

Figure 111 and Figure 112 show examples of the frequency

translation stage for both real and complex inputs, respectively.

BANDWIDTH OF

INTEREST

BANDWIDTH OF

INTEREST IMAGE

NCO FREQUENCY TUNING WORD (FTW) SELECTION

48-BIT NCO FTW = MIXING FREQUENCY/ADC SAMPLE RATE × 248

ADC + DIGITAL MIXER + NCO

REAL INPUT—SAMPLED AT

–

fS/32

–

fS/32

cos(ωt)

0°

90°

ADC

SAMPLING

REAL

–sin(ωt)

48-BIT

NCO

POSITIVE FTW VALUES

NEGATIVE FTW VALUES

COMPLEX

–6dB LOSS DUE TO

NCO + MIXER

–

fS/2

–

fS/3

–

fS/4

–

fS/8

fS/16

fS/8

fS/4

fS/3

fS/2

–

fS/16

–

fS/32

48-BIT NCO FTW =

ROUND ((

fS/3)/fS × 248) = +9.382513

(0x5555_5555_5555)

48-BIT NCO FTW =

ROUND ((

fS/3)/fS × 248) = –9.382513

(0xAAAA_AAAA_AAAA)

Figure 111. DDC NCO Frequency Tuning Word Selection—Real Inputs

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