AD9753ASTZ datasheet(19/28 Pages) AD

19 / 28 page

REV. B

AD9753

–19–

–30

START 100kHz

–40

–50

–60

–70

–80

–90

–100

–110

–120

–130

12.49MHz/

STOP 125MHz

1RM

–74.34dBm

+9.71442886MHz

–76.86dBm

–80.96dBm

–11.92dBm

1 [T1]

CH PWR

ACP UP

ACP LOW

C11

Cu1

C11

Cu1

MARKER 1 [T1]

RBW

5kHz

RF ATT

0dB

–74.34dBm

VBW

50kHz

9.71442886MHz

SWT

12.5 s

UNIT

dBm

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 adja-

cent channel. In Figure 28, the adjacent channel power ratio

(ACPR) at the output of the AD9753 is measured to be 65 dB.

The limitation 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, the

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 degrading 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.

AMPLITUDE (dBFS)

–10

–20

–15

–5

Figure 29. ACPR vs. Amplitude for QAM Carrier

DAC

INPUT

LATCHES

INPUT

LATCHES

PLL/DIVIDER

CLK+ CLK– PLLLOCK

DVDD AVDD

IOUTA

IOUTB

PORT 1

DATA

INPUT

PORT 2

DATA

INPUT

RSET2

1.9k

FSADJ

0.1 F

REFIO ACOM1 ACOM

DCOM

AD9753

0.1 F

INPM

INPP

LOIM

LOIP

OUTP

OUTM

AD8343 ACTIVE MIXER

0.1 F

LOINPUT

M/A-COM ETC-1-1-13 WIDEBAND BALUN

Figure 30. QAM Transmitter Architecture Using AD9753 and AD8343 Active Mixer

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