AD9142BCPZ datasheet(28/65 Pages) AD

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AD9142

Data Sheet

Rev. 0 | Page 28 of 64

Table 16. Half-Band Filter 2 Coefficient

Lower Coefficient

Upper Coefficient

Integer Value

H(1)

H(23)

−2

H(2)

H(22)

H(3)

H(21)

+17

H(4)

H(20)

H(5)

H(19)

−75

H(6)

H(18)

H(7)

H(17)

+238

H(8)

H(16)

H(9)

H(15)

−660

H(10)

H(14)

H(11)

H(13)

+2530

H(12)

+4096

Table 17. Half-Band Filter 3 Coefficient

Lower Coefficient

Upper Coefficient

Integer Value

H(1)

H(11)

+29

H(2)

H(10)

H(3)

H(9)

−214

H(4)

H(8)

H(5)

H(7)

+1209

H(6)

+2048

DIGITAL MODULATION

The AD9142 provides two modes to modulate the baseband

quadrature signal to the desired DAC output frequency.

•

Coarse (fS/4) modulation

•

Fine (NCO) modulation

fS/4 Modulation

The fS/4 modulation is a convenient and low power modulation

mode to translate the input baseband frequency to a fixed fS/4

IF frequency, fS being the DAC sampling rate. When modulation

frequencies other than this frequency are required, the NCO

modulation mode must be used.

NCO Modulation

The NCO modulation mode makes use of a numerically

controlled oscillator (NCO), a phase shifter, and a complex

modulator to provide a means for modulating the signal by a

programmable carrier signal. A block diagram of the digital

modulator is shown in Figure 45. The NCO modulation allows

the DAC output signal to be placed anywhere in the output

spectrum with very fine frequency resolution.

Figure 45. NCO Modulator Block Diagram

The NCO modulator mixes the carrier signal generated by the

NCO with the I and Q signals. The NCO produces a quadrature

carrier signal to translate the input signal to a new center fre-

quency. A complex carrier signal is a pair of sinusoidal waveforms

of the same frequency, offset 90 degrees from each other. The

frequency of the complex carrier signal is set via NCO_FREQ_

TUNING_WORD[31:0] in Register 0x31 through Register 0x34.

The NCO operating frequency, fNCO, is always equal to fDAC, the

DACCLK frequency. The frequency of the complex carrier

signal can be set from dc up to ±0.5 × fNCO.

The frequency tuning word (FTW) is in twos complement

format. It can be calculated as

DAC

CARRIER

DAC

≤

−

( )(

)

≥

CARRIER

DAC

CARRIER

FTW

(

)

(

)

(

−

CARRIER

DAC

CARRIER

FTW

The generated quadrature carrier signal is mixed with the I and

Q data. The quadrature products are then summed into the I

and Q data paths, as shown in Figure 45.

Updating the Frequency Tuning Word

The frequency tuning word registers are not updated immediately

upon writing, as are other configuration registers. Similar to

FIFO reset, the NCO update can be triggered in two ways.

•

SPI initiated update

•

Frame initiated update

COSINE

SINE

I DATA IN

Q DATA IN

I DATA OUT

Q DATA OUT

FTW[31:0]

NCO

PHASE[15:0]

AD9142BCPZ Datasheet(PDF) 28 Page - Analog Devices

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