AD9775 datasheet(31/56 Pages) AD

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AD9775

Rev. E | Page 31 of 56

It is possible to invert the I and Q selection by setting control

illustrates the timing requirements for the data inputs as well as

the IQSEL input. Note that the 1× interpolation rate is not

available in the one-port mode.

One-port mode is very useful when interfacing with devices

such as the Analog Devices AD6622 or AD6623 transmit signal

processors, in which two digital data channels have been

interleaved (multiplexed).

The programmable modes’ ONEPORTCLK inversion,

ONEPORTCLK driver strength, and IQ pairing described in

the PLL Enabled, One-Port Mode section have identical

functionality with the PLL disabled.

DIGITAL FILTER MODES

The I and Q datapaths of the AD9775 have their own

independent half-band FIR filters. Each datapath consists of

three FIR filters, providing up to 8× interpolation for each

channel. The rate of interpolation is determined by the state of

Control Register

0x01, Bit 7 and Bit 6. Figure 2 to Figure 4 show

the response of the digital filters when the AD9775 is set to 2×,

4×, and 8× modes. The frequency axes of these graphs are

normalized to the input data rate of the DAC. As the graphs

show, the digital filters can provide greater than 75 dB of

out-of-band rejection.

An online tool is available for quick and easy analysis of the

AD9775 interpolation filters in the various modes. The link can

be accessed at www.analog.com/ad9777image.

AMPLITUDE MODULATION

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

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

such that the waveform that leads in phase is cosinusoidal and

the waveform that lags is sinusoidal. Analysis of complex

variables states that the cosine waveform can be defined as

having real positive and negative frequency components, while

the sine waveform consists of imaginary positive and negative

frequency images. This is shown graphically in the frequency

domain in Figure 57.

e–jωt/2j

e–jωt/2

COSINE

SINE

Figure 57. Real and Imaginary Components of

Sinusoidal and Cosinusoidal Waveforms

Amplitude modulating a 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 reduces the positive and negative frequency images by a

factor of 2.

This scaling is very important in the discussion of the various

modulation modes. The phase relationship of the modulated

signals is dependent on whether the modulating carrier is

sinusoidal or cosinusoidal, again with respect to the reference

point of the viewer. Examples of sine and cosine modulation are

given in Figure 58.

SINUSOIDAL

MODULATION

COSINUSOIDAL

MODULATION

Ae–jωt/2j

Ae–jωt/2

Figure 58. Baseband Signal, Amplitude Modulated

with Sine and Cosine Carriers

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