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PDF ML6674 Data sheet ( Hoja de datos )
| Número de pieza | ML6674 | |
| Descripción | ATM 155Mbps UTP Transceiver | |
| Fabricantes | Micro Linear | |
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July 1999
ML6674
ATM 155Mbps UTP Transceiver
GENERAL DESCRIPTION
The ML6674 is a complete monolithic transceiver for
155Mbps NRZ encoded data transmission over category
5 unshielded twisted pair (UTP) and shielded twisted pair
(STP) cables. The ML6674 is compliant with the ATM
155Mbps Twisted Pair Specification. The ML6674 includes
the baseline restoration function and adaptive
equalization which will accurately compensate for line
losses exceeding 100m of UTP.
The ML6674 receive section consists of an equalizing
filter with a feedback loop for controlling effective line
compensation. The feedback loop contains a filter and
detection block for determining the proper control signal.
An ECL 100K compatible buffer at the output interfaces
directly with ATM physical interface chips.
The ML6674 transmit section accepts ECL 100K
compatible NRZ inputs.
FEATURES
s Complies with ATM Forum 155Mbps twisted pair
specification
s Integrated baseline wander correction circuit
s Integrated adaptive equalization
s Transmitter can be externally turned off
(high impedence) for true quiet line
s Operates over 100 meters of STP or category 5 UTP
Twisted Pair Cable
s 32-pin PLCC and TQFP
BLOCK DIAGRAM
TXIN+
TXIN–
LPBK
TXOFF
TVCCA
TVCCD
RTSET
RTSET1
RTSET2
TPOUT+
TPOUT–
SD+
SD–
RXOUT+
RXOUT–
MUX
TGNDA TGNDD
LINK
STATUS
ADAPTIVE
CONTROL
ADAPTIVE
CONTROL REFERENCE
ADAPTIVE
EQUALIZER
TPIN+
TPIN–
RGND
RRSET1
RRSET2
RRSET RVCCA RVCCD CMREF
1
1 page  
FUNCTIONAL DESCRIPTION
The ML6674 transceiver is a physical media dependent
transceiver that allows the transmission and reception of
155 Mbps data over 100 meters over shielded twisted pair
cable or category 5 unshielded twisted pair cable.
The transmit section accepts NRZ data, sending the
information on a two pin current driven transmitter. The
transmitted output passes through an external low pass
filter and transformer before entering the connectors to the
STP or UTP cable. The output amplitude of the transmitted
signal is programmable through the external RTSET
resistor.
The receive section accepts NRZ coded data after it
passes through an isolation transformer and band limiting
filter. The adaptive equalizer is used to compensate for
the amplitude and phase distortion incurred from the
cable. The adaptive control section determines the signal
amplitude (and therefore the cable length) and adjusts the
equalizer accordingly. As the input signal amplitude
diminishes, the amount of equalization increases until it
reaches its maximum (Figure 1).
The receiver also includes the Baseline Wander correction
circuitry. The circuit will compensate and track the DC
baseline wander caused by DC imbalance of the received
data. A 10pF capacitor can be connected between TPIN+
and TPIN– to improve Bit Error Rate.
The adaptive control block governs both the equalization
level as well as the signal detection status. Signal detect
is asserted when the equalizer control loop settles, or
when loop back is asserted. When the input signal is
small, the equalization will be at its maximum.
After the signal has been equalized, it is fed through the
loopback multiplexer onto the RXOUT± pins.
Figure 1 shows a typical gain vs frequency plot of the
adaptive equalizer for 0, 25, 50, 75 and 100 meter
category 5 cable lengths.
TRANSMISSION
PECL level scrambled NRZ data is received by the
ML6674 and the current driven transmitter then sent the
data to the filter/transformer module. The transmit
amplitude is controlled by one external resistor, RTSET.
IOUT
=
32 × 1.25V
RTSET
For ATM UTP applications the transmit amplitude is 1V
peak to peak. The termination at the transmitter output is
50ý. Therefore the transmit current IOUT = 1/50 = 20 mA.
Thus,
RTSET = 32 × 1.25V = 2kΩ
20mA
The transmitter may be disabled via the TXOFF pin. When
this pin is pulled low, the transmitter’s output goes to a
ML6674
high impedance state and no current flows through the
transformer.
ADAPTIVE EQUALIZATION
During transmission of data over UTP (unshielded twisted
pair), distortion and ISI are caused by dispersion in the
cable. Equalization is used to overcome this signal
corruption. However, the distortion is frequency
dependent and cable length dependent. Therefore, in most
practical cases, the TP port characteristic is unknown and
it is impractical to tune the equalizer specifically to each
individual port. Hence, adaptive equalizer is used in the
TP-PMD to ensue proper compensation of the received
signal.
By using adaptive equalizer, the receiver automatically
compensates different length of cable without over
equalizing or under equalizing the line. The ML6674
monitors the amplitude of the received signal to
determine the cable length and adjust the equalizer
accordingly. The input signal level is inversely
proportional to the cable length. Therefore, as the signal
level decreases, the amount of equalization is increased
to compensate for the line loss.
ML6672 COMPATIBILITY
The ML6674 implements the Baseline Wander correction
circuit, in addition to providing the functionality of the
existing ML6672 device. The ML6674 is plug-compatible
with the ML6672 with the following notes:
• In the ML6674 design, the RTSET resistor must be
2.0kW
• In the ML6674 design, the following passive
components used in the ML6672 design may be
eliminated from the ML6674 design
— RSET resistor
— RTH resistor
— CAP1 capacitor
— CAP2 capacitor
20
15
10
5
0 1 x 106
1 x 107
1 x 108
Figure 1. Equalization Range
1 x 109
5
5 Page | ||
| Páginas | Total 8 Páginas | |
| PDF Descargar | [ Datasheet ML6674.PDF ] | |
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