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MTC20154

MTC20154 RX PATH

Speech Filter

An external bidirectional LP filter for up and downstream POTS service splits out the speech signal to the

analog telephone circuit on both the NT and LT sides of the line. The ADSL analog front end integrated

circuit does not contain any circuitry for the POTS service but guarantees that the POTS bandwidth is not

disturbed by spurious signals from the ADSL spectrum.

Channel Filters

The purpose of these external analog circuits is to provide partial echo cancellation by analog filtering of

the receive signal for both ATU-R (reception of downstream channel) and ATU-C (reception of upstream

channel). This is feasible because the upstream and the downstream data can be modulated on separate

carriers (FDM).

Signal Attenuator (ATT) and Low Noise Amplifier

The attenuator needs to be DC decoupled from the external circuitry. In fact, it is also used to internally fix

the LNA input common mode voltage at the nominal value: AVDD/2. This is done by the use of an internal

biasing circuit. It is therefore mandatory to decouple the MTC20154 input from any external DC biasing

system. The Low Noise Amplifier (LNA) placed after the ATT will be used in combination with the attenu-

ation block. The goal is to obtain a range of RX path input level varying from –15 dB to 31 dB, while main-

taining the noise contribution negligible.

Figure 6. Signal Attenuator (ATT) and Low Noise Amplifier

The input attenuator will have the following characteristics

Table 9. Attenuator Characteristics

Data

Min

Typical

Max

Unity

Input type

AC only, DC decoupled

–

Common mode voltage (forced)

–

VDD/2

–

V

Maximum input differential voltage

–

VDD

VDD+1Vd

Vpdiff

Input impedance

10

14.47

19

KOhms

Attenuation step

–

5

–

dB

Attenuation range

0

–

–

15

dB

Maximum attenuation error

–

.05

–

dB

Digital interface

–

2

–

bits

Digital code

‘11’ –> 0 dB attenuation

’00’ –> –15 dB attenuation

Gain

RXP

RXN

Attenuator

MTC-20154

Vref

5

Attenuation

2

LNA