AD9773AST datasheet(16/19 Pages) AD

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AD9773

PRELIMINARY TECHNICAL DATA

interpolation rate.The input data rate must also match

this clock speed. Note that in this mode, the data rate at

the input to the interpolation filters is the same as the

input data rate at ports one and two.

PLL DISABLED, ONE PORT MODE

The one port mode is selected by setting control reg-

ister 02h, bit 6, to logic 1. Data to the I and Q channels

must now be multiplexed onto the data entering data

port 1. Pin 32 (ONEPORTCLK) is now a clock signal

output . Because the multiplexed data must run at twice

the data rate of the inputs to the I and Q channels, the

speed of ONEPORTCLK is defined as 2

× the speed of

the clock at CLK+/CLK-, divided by the interpolation

rate. Pin 31 (IQSEL) can be used to select the I or Q

channels for input. IQSEL =1, followed by a rising

clock edge will latch the input data into the I channel,

while IQSEL =0, followed by a rising clock edge will

latch the input data into the Q channel.

One port mode is very useful when interfacing with

devices, such as the Analog Devices AD6622Transmit

Signal Processor, in which two digital data channels

have been interlaced (multiplexed).

As defined in control register 02h, bit 7, the AD9773

can accept either signed or unsigned input data.

DIGITAL FILTER MODES

The I and Q data paths of the AD9773 each have their

own independent half-band FIR filters, providing up to

× interpolation for each channel. Each channel consists

of 3 FIR filters. Figure 1 shows the response of the

digital filters when the AD9773 is set to 2

×, 4×, and 8×

modes. Note that the frequency axis of these graphs

have been normalized to the output data rate of the

DAC. As the graphs show, the digital filters can provide

greater than 75dB of out of band rejection.

MODULATION MODES

INTERPOLATING (NO MODULATION)

With control register 01h, bits 5 and 4, set to 00, the

digital modulators on the AD9773 are disabled.The

AD9773 operates in this mode simply as a dual interpo-

lating (1

×, 2×, 4×, 8×) DAC. Filter responses for this

mode are defined in Figure 1.

INTERPOLATING (REAL MIX MODULATION)

The digital modulators in the AD9773 can be enabled

by setting control register 01h, bits 5 and 4, to corre-

spond to the desired fs/2, fs/4, fs/8 modulation mode

(see register descriptions on page 11). Real mix mode is

enabled by setting control register 01h, bit 2, to a logic

1. In this mode, the modulators act individually on each

data path, with no complex mixing between modulators.

Figure 9. Real and Imaginary Components of Sinusoidal

and Cosinusoidal Waveforms.

INTERPOLATING (COMPLEX MIX MODE)

Complex Modulation is enabled by setting control

digital modulators on the AD9773 are coupled to

provide a complex modulation function. In conjunction

with an external quadrature modulator, this complex

modulation can be used to realize a transmit image

rejection architecture.The complex modulation function

can be programmed for e+jωt or e-jωt to give upper or

lower image rejection.The modulation frequency

ω can

be programmed via the SPI port for fs/2, fs/4 and fs/8,

where fs represents the DAC output rate.

AMPLITUDE MODULATION

Given two sine waves at the same frequency, but with a

90 phase difference, a point of view in time can be taken

such the waveform which leads in phase is cosinusoidal,

and the waveform which lags is sinusoidal. Analysis of

complex variables states that the cosine waveform can

then be defined with real positive and negative fre-

quency components, while the sine waveform consists of

imaginary positive and negative frequency components.

These waves are shown graphically in the frequency

domain in figure 9.

Amplitude modulating a real baseband signal with a

sine or a cosine convolves the baseband signal with the

modulating carrier in the frequency domain. Amplitude

scaling of the modulated signal occurs and is dependent

on whether the modulating carrier is sine or cosinusoid-

al, again with respect to the reference point of the

viewer. An example of sine and cosine modulation is

given in figure 10.

OPERATIONS ON COMPLEX SIGNALS

Truly complex signals can not be realized outside of a

computer simulation. However, two data channels, both

consisting of real data, can be defined as the real and

imaginary components of a complex signal. I (real) and

Q (imaginary) data paths are often defined this way. By

ejωt/2

e-jωt/2

cosine

ejωt/2j

e-jωt/2j

sine

REV. PrA

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