AD9753AST datasheet(20/28 Pages) AD

20 / 28 page

REV. B

–20–

AD9753

–20

CENTER 860MHz

–30

–40

–50

–60

–70

–80

–90

–100

–110

–120

11MHz/

SPAN 110MHz

2MA

MARKER 1 [T2]

RBW

10kHz

RF ATT

0dB

–99.88dBm

VBW

10kHz

859.91983968MHz

SWT

2.8 s

UNIT

dBm

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

–99.88bBm,

+859.91983968MHz

–65.67dBm

–65.15dBm

–7.05dBm

33.10dB

–49.91983968MHz

33.10dB

–49.91983968MHz

1 [T2]

CH PWR

ACP UP

ACP LOW

1 [T2]

2 [T2]

C11

Cu1

Figure 31. Signal of Figure 28 Mixed to Carrier

Frequency of 800 MHz

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

Textbook analyses of Bit Error Rate (BER) performance are

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 quadra-

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

Figure 32 implies gray 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 neces-

sary. To calculate energy per symbol, we 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 strongest

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

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

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

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

will dominate the BER calculation.

The AD9753 has a worst-case in-band SFDR of 47 dB at the

upper end of its frequency spectrum (see TPCs 4 and 7). 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)

1E–0

1E–3

1E–6

1E–2

1E–5

1E–1

1E–4

16 QAM

64 QAM

4 QAM

Figure 32. Probability of a Symbol Error for QAM

Pseudo Zero Stuffing/IF Mode

The excellent dynamic range of the AD9753 allows its use in

applications where synthesis of multiple carriers is desired. In

addition, the AD9753 can be used in a pseudo zero stuffing

mode that improves dynamic range at IF frequencies. In this

mode, data from the two input channels is interleaved to the

DAC, which is running at twice the speed of either of the input

ports. However, the data at Port 2 is held constant at midscale.

The effect of this is shown in Figure 33. The IF signal is the

image, with respect to the input data rate, of the fundamen-

tal. Normally, the sinx/x response of the DAC will attenuate

this image. Zero stuffing improves the pass-band flatness so that

the image amplitude is closer to that of the fundamental sig-

nal. Zero stuffing can be an especially useful technique in the

synthesis of IF signals.

FREQUENCY (Normalized to Input Data Rate)

–30

0.5

–20

–50

–10

–40

1.5

AMPLITUDE

OF IMAGE

WITHOUT

ZERO STUFFING

AMPLITUDE

OF IMAGE

USING

ZERO STUFFING

Figure 33. Effects of Pseudo Zero Stuffing on

Spectrum of AD9753

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