MTC20454 ST Microelectronics, Inc., MTC20454 Datasheet - Page 8

• Manufacturer Part Number: MTC20454
• Description: Quad Integrated Adsl CMOS Analog Front End Circuit
• Manufacturer: ST Microelectronics, Inc.

MTC20454
MTC20454RX PATH
Speech Filter
An external bi-directional LP filter for up and downstream POTS service splits out the speech signal to the
analog telephone. 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. This is feasible because the upstream and the downstream data can be modulated on sepa-
rate 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 de-couple the MTC20454
input from any external DC biasing system. The Low Noise Amplifier (LNA) placed after the ATT will be
used in combination with the attenuation block.
The goal is to obtain a range of RX path input level varying from –15 dB to 31 dB, while maintaining the
noise contribution negligible.
RX Filters
The combination of the external filter (an LC ladder filter typically) with the integrated low pass filter pro-
vides:
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■
Linearity of RX
Linearity of the RX analog path is defined by the IM3 product of two sinusoidal signals with frequencies f1
and f2 and each with 0.5 Vpd amplitude (total _1 Vpd) at the output of the RX-AGC amplifier (i.e: before
the ADC) for the case of minimal AGC setting.
Figure 5. Signal Attenuator (ATT) and Low Noise Amplifier
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echo reduction to improve dynamic Range - DMT sidelobe and out of band (anti-aliasing) attenuation.
Anti alias filter (60 dB rejection @ image freq.)
RXP
RXN
V ref
Attenuator
Attenuation
2
Gain
LNA
5
MTC 20454