AC206 View Datasheet(PDF) - Broadcom Corporation
Part Name
Description
MFG CO.
'AC206' PDF : 70 Pages View PDF
Preliminary Data Sheet
07/08/02
AC206
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 10Ω 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.
Document AC206-DS05-405-R
Broadcom
Receive Function Page 3
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