AC208-DS02-R datasheet(11/58 Pages) BOARDCOM

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Preliminary Data Sheet

AC208

07/08/02

B roa dcom

Document

AC208-DS04-405-R

Receive Function

Page 3

MULTIMODE TRANSMIT DRIVER

The multimode driver transmits MLT-3 coded signal in 100BASE-TX mode and Manchester coded signal in 10BASE-T

mode.

The slew rate of the transmitted MLT-3 signal can be controlled to eliminate high frequency EMI component. The MLT-3

signal after the magnetic has a typical rise/fall time of approximately 4 ns, which is within the target range specified in the

ANSI TP- PMD standard.

In 10BASE-T mode, high frequency pre-emphasis is performed which extends the cable-driving distance without the need

of an external filter. FLP/NLP also drives signals through the 10BASE-T driver. The 10BASE-T and 100BASE-TX transmit

signals are multiplexed to the transmit output driver. This arrangement results in using the same external transformer for

both the 10BASE-T and the 100BASE-TX. The driver output level is set by a built-in bandgap reference and an external

resistor connected to the RIBB output pin. The resistor sets the output current for all modes of operation. Each of the TXOP/

N outputs is an open drain device which has a source resistance of 10

Ω maximum and a current rating of 40 mA for the 2

Vp-p MLT-3 signal, 100 mA for 5Vp-p Manchester signal when used 1:1 transformer.

PLL CLOCK SYNTHESIZER

The Transceiver also includes on-chip PLL clock synthesizer that generates a 125-MHz and a 25-MHz clock for the

100BASE-TX, or a 100-MHz and 20-MHz clock for the 10BASE-T and auto-negotiation operations. The PLL clock generator

uses a fully differential VCO cell that induces a very low jitter. The Zero Dead Zone Phase Detection method implemented

in this design provides excellent phase tracking. A charge pump with charge sharing compensation is also included to further

reduce jitter at different loop filter voltages. On-chip loop filter eliminates the need for external components and avoids

external noise pickup. Only one external 25-MHz crystal or a signal source is required as a reference clock.

RECEIVE FUNCTION

In 100BASE-TX mode, the receive function implements the reverse order function in the transmit path. It includes a receiver

with adaptive equalization and DC restoration, MLT-3 to NRZI conversion, data and clock recovery at 125 MHz, NRZI to NRZ

conversion, Serial-to-Parallel conversion, de-scrambling, and 5B to 4B decoding. The receiver circuit starts with a DC bias

for the differential RX± inputs, follows with a low-pass filter to filter out high frequency noise from the transmission channel

media. An energy detect circuit is also added to determine whether there is any signal energy on the media. This is useful

in the power-saving mode. The amplification ratio and slicer threshold is set by the on-chip bandgap reference.

In 10BASE-T mode, signal first passes through a third order lowpass filter, which filters all the noise from the cable, board,

and transformer. This eliminates the need for a 10BASE-T external filter. A Manchester decoder and a Serial-to Parallel

follows to generate the 4-bit data in MII mode.

ADAPTIVE EQUALIZER

Each of the eight transceivers is designed to accommodate for maximum cable length of 150m UTP CAT5 cable. A 150m

of UTP CAT-5 cable (such as AT&T 1061) has an attenuation of 31 dB at 100 MHz. A typical attenuation of a 100m cable is

20 dB. The worst case attenuation is around 24–26 dB defined by TP-PMD.

The amplitude and phase distortion from the cable causes inter-symbol interference (ISI) which makes clock and data

recovery impossible. Adaptive equalizer is done by matching the inverse transfer function of the twist-pair cable. This is a

variable equalizer that changes its equalizer frequency response in accordance to cable length. The cable length is

estimated based on comparisons of incoming signal strength against some the known cable characteristics. The equalizer

has a monotonically frequency response, and tunes itself automatically for any cable length to compensate for the amplitude

and phase distortion incurred from the cable.

AC208-DS02-R 数据表(PDF) 11 Page - Broadcom Corporation.

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