AD9751AST datasheet(18/26 Pages) AD

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REV. 0

AD9751

–18–

APPLICATIONS

QAM/PSK Synthesis

Quadrature modulation (QAM or PSK) consists of two baseband

PAM (Pulse Amplitude Modulated) data channels. Both chan-

nels are modulated by a common frequency carrier. However,

the carriers for each channel are phase-shifted 90

° from each

other. This orthogonality allows twice the spectral efficiency

(data for a given bandwidth) of digital data transmitted via AM.

Receivers can be designed which selectively choose the ‘in phase’

and ‘quadrature’ carriers, and then recombine the data. The

recombination of the QAM data can be mapped as points repre-

senting digital words in a two dimensional constellation as shown

in Figure 27. Each point, or symbol, represents the transmission

of multiple bits in one symbol period.

0100

0101

0001

0000

0110

0111

0011

0010

1110

1111

1011

1010

1100

1101

1001

1000

Figure 27. 16 QAM Constellation, Gray Coded (Two 4-Level

PAM Signals with Orthogonal Carriers)

Typically, the I and Q data channels are quadrature-modulated

in the digital domain. The high data rate of the AD9751 allows

extremely wide band (>10 MHz) quadrature carriers to be syn-

thesized. Figure 28 shows an example of a 25 MSymbol/S

QAM signal, oversampled by eight at a data rate of 200 MSPS;

modulated onto a 25 MHz carrier and reconstructed using the

AD9751. The power in the reconstructed signal is measured

to be –12.08 dBm. In the first adjacent band, the power is

–73.67 dBm, while in the second adjacent band the power is

–76.91 dBm.

–30

START 100kHz

–40

–50

–60

–70

–80

–90

–100

–110

–120

–130

12.49MHz/

STOP 125MHz

1RM

–74.49bBM,

+9.71442886MHz

–73.67dBm

–76.91dBm

–12.08dBm

1 [T1]

CH PWR

ACP UP

ACP LOW

C11

Cu1

C11

Cu1

MARKER 1 [T1]

RBW

5kHz

RF ATT

0dB

–74.49dBm

VBW

50kHz

9.71442886MHz

SWT

12.5 s

UNIT

dBm

COMMENT A: 25 MSYMBOL, 64 QAM, CARRIER = 25MHz

Figure 28. Reconstructing Raised Cosine Signal at

120 MHz IF

A figure of merit for wideband signal synthesis is the ratio of signal

power in the transmitted band to the power in an adjacent chan-

nel. In Figure 29, the adjacent channel power ratio (ACPR) at

the output of the AD9751 is measured to be 62 dB. The limita-

tion on making a measurement of this type is often not the DAC

but the noise inherent in creating the digital data record using

computer tools. To find how much this is limiting the perceived

DAC performance, the signal amplitude can be reduced, as is

shown in Figure 29. The noise contributed by the DAC will

remain constant as the signal amplitude is reduced. When the

signal amplitude is reduced to the level where the noise floor

drops below that of the spectrum analyzer, ACPR will fall off at

the same rate that the signal level is being reduced. Under the

conditions measured in Figure 28, this point occurs in Figure 29

at –4 dBFS. This shows that the data record is actually degrad-

ing the measured ACPR by up to 4 dB.

AMPLITUDE – dBFS

–10

–20

–15

–5

Figure 29. ACPR vs. Amplitude for QAM Carrier

A single-channel active mixer such as the Analog Devices AD8343

can then be used for the hop to the transmit frequency. Figure 30

shows an applications circuit using the AD9751 and the AD8343.

The AD8343 is capable of mixing carriers from dc to 2.5 GHz.

Figure 31 shows the result of mixing the signal in Figure 28 up

to a carrier frequency of 800 MHz. ACPR measured at the

output of the AD8343 is shown in Figure 31 to be 58 dB.

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