AD9154 datasheet(63/124 Pages) AD

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Data Sheet

AD9154

Rev. C | Page 63 of 124

Filter Performance Beyond Specified Bandwidth

The usable pass band of the interpolation filter is specified as

0.4 × fDATA. The filters can be used slightly beyond this ratio at

the expense of increased pass-band ripple and decreased

interpolation image rejection.

–0.6

–0.5

–0.4

–0.3

–0.2

–0.1

BANDWIDTH (%

fDATA)

PASS-BAND RIPPLE

IMAGE REJECTION

Figure 70. Interpolation Filter Performance Beyond Specified Bandwidth

Figure 70 shows the performance of the interpolation filters

beyond 0.4 × fDATA. Note that the ripple increases much slower

than the image rejection decreases. This means that if the

application can tolerate degraded image rejection from the

interpolation filters, more bandwidth can be used.

DIGITAL MODULATION

The AD9154 includes modulation blocks that upconvert I/Q

quadrature signal pairs to an IF frequency in the digital domain.

The coarse modulation modes (fDAC/4 and fDAC/8) upconvert an

I/Q pair of digital signals to one of the selected IFs. The NCO

fine modulation mode upconverts an I/Q signal pair to an IF

frequency programmed into the NCO. Modulation mode is

selected as shown in Table 64 and is paged as described in the

Dual Paging section.

Table 64. Modulation Mode Selection

Modulation Mode

MODULATION_TYPE

None

0b00

NCO Fine Modulation

0b01

Coarse − fDAC/4

0b10

Coarse − fDAC/8

0b11

NCO Fine Modulation

This modulation mode uses the NCO, a phase shifter, and a

complex modulator to upconvert an I/Q digital signal pair to an

IF frequency within the first Nyquist zone of the DAC cores.

Figure 71 shows a block diagram of the NCO modulator. This

allows output signals to be placed anywhere in the output

spectrum with very fine frequency resolution. The NCO

produces a quadrature carrier to translate the input signal to a new

center frequency. A quadrature carrier is a pair of sinusoidal

waveforms of the same frequency, offset 90° from each other.

The frequency of the quadrature carrier is set via an FTW. The

quadrature carrier is mixed with the I and Q data and then

summed into the I and Q datapaths, as shown in Figure 71.

−fDAC/2 ≤ fCARRIER < +fDAC/2

FTW = (fCARRIER/fDAC) × 248

where FTW is a 48-bit twos complement number.

The frequency tuning word is set as shown in Table 65 and

paged as described in the Dual Paging section.

Table 65. NCO FTW Registers

Address

Value

Description

0x114

FTW[7:0]

8 LSBs of FTW

0x115

FTW[15:8]

Next 8 bits of FTW

0x116

FTW[23:16]

Next 8 bits of FTW

0x117

FTW[31:24]

Next 8 bits of FTW

0x118

FTW[39:32]

Next 8 bits of FTW

0x119

FTW[47:40]

8 MSBs of FTW

Unlike other registers, the FTW registers are not updated

immediately upon writing. Instead, the FTW registers update

on the rising edge of FTW_UPDATE_REQ (Register 0x113[0]).

After an update request, FTW_UPDATE_ACK (Register 0x113[1])

must be high to acknowledge that the FTW has updated.

SEL_SIDEBAND (Register 0x111, Bit 1; paged as described in

the Dual Paging section) is a convenience bit that can be set to

use the negative modulation result. This is equivalent to flipping

the sign of FTW.

INTERPOLATION

NCO

–1

COS(ωn + θ)

SIN(ωn + θ)

SINE

I DATA

Q DATA

FTW[47:0]

SPECTRAL

INVERSION

OUT_I

OUT_Q

–

NCO PHASE OFFSET

[15:0]

Figure 71. NCO Modulator Block Diagram

NCO Phase Offset

The NCO phase offset feature allows rotation of the I and Q

phases. Unlike phase adjust, this feature moves the phases of

both I and Q channels together. NCO phase offset can be used

only when using NCO fine modulation.

−180° ≤ DegreesOffset < +180°

PhaseOffset = (DegreesOffset/180°) × 215

where PhaseOffset is a 16-bit twos complement number.

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