AD6684 datasheet(38/107 Pages) AD

38 / 107 page

AD6684

Data Sheet

Rev. 0 | Page 38 of 107

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 75 and Figure 76 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

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

ADC + DIGITAL MIXER + NCO

REAL INPUT—SAMPLED AT

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

48-BIT NCO FTW =

ROUND ((

fS/3)/fS × 248) =

+9.382513 (0x555555555555)

48-BIT NCO FTW =

ROUND ((–

fS/3)/fS × 248) =

–9.382513 (0xFFFF000000000000)

–

fS/2

–

fS/4

–

fS/8

fS/16

fS/8

fS/4

fS/2

–

fS/16

–

fS/32

–

fS/3

–

fS/32

–

fS/32

Figure 75. DDC NCO Frequency Tuning Word Selection—Real Inputs

AD6684 Datasheet(PDF) 38 Page - Analog Devices

AD6684 Datasheet(HTML) 38 Page - Analog Devices

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 ...More

Datasheet Download

Link URL