AD9753-EB(Rev0) View Datasheet(PDF) - Analog Devices

Part Name

Description

Manufacturer

AD9753-EB
(Rev.:Rev0)

12-Bit, 300 MSPS High-Speed TxDAC+® D/A Converter

Analog Devices 

AD9753

APPLICATIONS

QAM/PSK Synthesis

Quadrature modulation (QAM or PSK) consists of two base-

band PAM (Pulse Amplitude Modulated) data channels. Both

channels are modulated by a common frequency carrier. How-

ever, the carriers for each channel are phase-shifted 90° from

each other. This orthogonality allows twice the spectral effi-

ciency (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 representing digital words in a two dimensional constella-

tion 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 AD9753 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

AD9753. The power in the reconstructed signal is measured to be

–11.92 dBm. In the first adjacent band, the power is –76.86 dBm

while in the second adjacent band, the power is –80.96 dBm.

MARKER 1 [T1]

RBW 5kHz RF ATT 0dB

–74.34dBm VBW 50kHz

9.71442886MHz SWT 12.5 s UNIT

dBm

–30

1 [T1]

–74.34dBM

–40

+9.71442886MHz

CH PWR

–76.86dBm

–50

ACP UP

–80.96dBm

ACP LOW

–11.92dBm

–60

1RM

–70 1

–80

–90

–100

–110

C11

–120

C11

C0

C0 Cu1

Cu1

–130

START 100kHz

12.49MHz/

STOP 125MHz

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

Figure 28. Reconstructed 64-QAM Signal at a 25 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 28, the adjacent channel power ratio (ACPR) at

the output of the AD9753 is measured to be 65 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 –10 dBFS. This shows that the data record is actually degrad-

ing the measured ACPR by up to 10 dB.

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 AD9753 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 59 dB.

80

70

60

50

40

–20

–15

–10

–5

0

AMPLITUDE – dBFS

Figure 29. ACPR vs. Amplitude for QAM Carrier

–18–

REV. 0

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