AD9751AST datasheet(19/26 Pages) AD

19 / 26 page

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AD9751

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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

AD9751

0.1 F

INPM

INPP

LOIM

LOIP

OUTP

OUTM

AD8343 ACTIVE MIXER

0.1 F

LOINPUT

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

Figure 30. QAM Transmitter Architecture Using AD9751 and AD8343 Active Mixer

–20

CENTER 860MHz

–30

–40

–50

–60

–70

–80

–90

–100

–110

–120

11MHz/

SPAN 110MHz

MARKER 1 [T2]

RBW

10kHz

RF ATT

0dB

–100.59dBm

VBW

10kHz

859.91983968MHz

SWT

2.8 s

UNIT

dBm

COMMENT A: 25 MSYMBOL, 64 QAM CARRIER @ 825MHz

Cu1

C11

2MA

–100.59bBm,

+859.91983968MHz

–64.88dBm

–62.26dBm

–7.38dBm

33.48dB

–49.91983968MHz

33.10dB

–49.91983968MHz

1 [T2]

CH PWR

ACP UP

ACP LOW

1 [T2]

2 [T2]

Figure 31. Signal of Figure 27 Mixed to Carrier

Frequency of 800 MHz

Effects of Noise and Distortion on Bit Error Rate (BER)

Textbook analysis of Bit Error Rate (BER) performance is

generally stated in terms of E (energy in watts-per-symbol or

watts-per-bit) and NO (spectral noise density in watts/Hz). For

QAM signals, this performance is shown graphically in Figure

32. M represents the number of levels in each quadrature PAM

signal (i.e., M = 8 for 64 QAM, M = 16 for 256 QAM). Figure

32 implies grey coding in the QAM constellation, as well as the

use of matched filters at the receiver, which is typical. The

horizontal axis of Figure 32 can be converted to units of energy/

symbol by adding to the horizontal axis 10 log of the number of

bits in the desired curve. For instance, to achieve a BER of 1e-6

with 64 QAM, an energy per bit of 20 dB is necessary. To

calculate energy per symbol, add 10 log(6), or 7.8 dB. 64 QAM

with a BER of 1e-6 (assuming no source or channel coding) can

therefore theoretically be achieved with an energy/symbol-to-

noise (E/NO) ratio of 27.8 dB. Due to the loss and interferers

inherent in the wireless path, this signal-to-noise ratio must be

realized at the receiver to achieve the given bit error rate.

Distortion effects on BER are much more difficult to determine

accurately. Most often in simulation, the energies of the stron-

gest distortion components are root-sum-squared with the noise,

and the result is treated as if it were all noise. That being said,

using the example above of 64 QAM with the BER of 1e-6, if

the E/NO ratio is much greater than the worst-case SFDR, the

noise will dominate the BER calculation.

The AD9751 has a worst-case in-band SFDR of 47 dB at the

upper end of its frequency spectrum (see TPCs 2, 3). When

used to synthesize high-level QAM signals as described above,

noise, as opposed to distortion, will dominate its performance

in these applications.

SNR/BIT – dB

–03

–06

–02

–05

–01

–04

16 QAM

64 QAM

4 QAM

Figure 32. Probability of a Symbol Error for QAM

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