TDA8046H 查看數據表（PDF） - Philips Electronics

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TDA8046H

Multi-mode QAM demodulator

Philips Electronics 

Philips Semiconductors

Multi-mode QAM demodulator

Product specification

TDA8046

7.1.2 EQUALIZER

This function is realized with a T spaced 12 or 14 taps

(selected via the I2C-bus) adaptive filter with a feedback

part. The equaliser is based on a Decision Feedback

Equalizer (DFE) structure with Least Mean Square (LMS)

coefficient updating algorithm. No training sequence is

required. The block schematic of the total equalizer is

shown in Fig.8. The main tap of the equalizer is adjustable

for fine AGC function (6 dB AGC range). The settings of

the equalizer taps can be read via the I2C-bus. If the

equalizer diverges, an alarm bit is set (I2C-bus bit ALEQ)

and an automatic reset of the taps can be performed

(I2C-bus bit EAR).

To improve acquisition time, the convergence steps of the

FFE/DFE parts of the equalizer are programmable via the

I2C-bus. When the system locks, the steps are

automatically modified for optimum performances.

Besides reading the equalizer tap values, the main tap of

the equalizer can also be programmed. After setting the

main tap, the other coefficients can be set to zero.

The equalizer settings can also be frozen via the I2C-bus.

The equalizer has been proven to work correctly under bad

channel conditions as indicated in Table 1. It is guaranteed

that all loops (including equalizer) converge at a SNR of

21 dB for a 64-QAM modulation format and 27 dB for a

256-QAM modulation format.

Table 1 Channel echo profile

DELAY

3⁄8 × Tsym

11⁄8 × Tsym

2 × Tsym

45⁄8 × Tsym

67⁄8 × Tsym

AMPLITUDE

0.08

0.20

0.05

0.10

0.03

PHASE

130°

60°

310°

200°

200°

Figure 7 represents the QAM spectrum seen by the

equalizer. It corresponds (in the frequency domain) to the

multiplication of a full nyquist spectrum by the impulse

response of the channel specified in Table 1.

1

handbook, full pagewidth

relative

gain

(dB)

−1

MGD636

−3

−5

−7

−9

−11

−0.5

−0.375

−0.25

−0.125

0

0.125

0.25

0.375

0.5

( ) relative frequency

f

rs

1996 Nov 19

Fig.7 QAM spectrum with echo profile as seen by the equalizer.

11

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