ML4664/ML4669
SYSTEM DESCRIPTION (Continued)
TPOUTP and TPOUTN, can drive a 100W, 150W load, or
a variety of impedances that are characteristic of the
twisted pair wire. RTSETTP selects the current into the
TPOUTP, TPOUTN pins. This current along with the
characteristic impedance of the cable determines the
output voltage.
While in the unsquelch state, the receive squelch circuit
looks for the start of idle signal at the end of the packet.
Start of idle occurs when the input signal remains idle for
more than 160ns. When start of idle is detected, the
receive squelch circuit returns to the squelch state and the
start of idle signal is output on the twisted pair outputs
TPOUTP, TPOUTN.
Once the characteristic impedance of the twisted pair is
determined, one must select the appropriate RTSETTP
resistor as well as match the terminating impedances of
the transmit and receive filter. The RTSETTP resistor can
be selected as follows:
INPUT AMPLIFIER
The OPINP, OPINN input signal is fed into a limiting
amplifier with a gain of about 100 and input resistance of
1.3kW. Maximum sensitivity is achieved through the use
of a DC restoration feedback loop and AC coupling the
input. When AC coupled, the input DC bias voltage is set
by an on-chip network at about 1.7V. These coupling
capacitors, in conjunction with the input impedance of
the amplifier, establish a high pass filter with 3dB corner
R
ꢀ ꢃ
L
RTSETTP =
220
ꢂ ꢅ
(2)
ꢁ100ꢄ
Where R is the characteristic impedance of the twisted
L
pair cable.
frequency, f , at:
L
The transmitter incorporates a pre-equalization circuit for
driving the twisted pair line. Pre-equalization
1
fL =
(3)
compensates for the amplitude and phase distortion
introduced by the twisted pair cable. The twisted pair line
will attenuate the 10MHz signal more than the 5MHz
signal. Therefore pre-equalization insures that both the 5
and 10MHz components will be roughly the same
amplitude at the far end receiver.
2 1300C
Since the amplifier has a differential input, two capacitors
of equal value are required. If the signal driving the input
is single ended, one of the coupling capacitors can be
tied to AV
.
CC
The pre-equalization circuit reduces the current output
when a 5MHz bit is being transmitted. After 50ns of a
5MHz bit, the current level is reduced to approximately
2/3 of its peak for the remaining 50ns. Figure 9 illustrates
the pre-equalization.
The internal amplifier has a lowpass filter built-in to band
limit the input signal which in turn will improve the
signal to noise ratio.
Although the input is AC coupled, the offset voltage
within the amplifier will be present at the amplifier’s
output. In order to reduce this error a DC feedback loop
is incorporated. This negative feedback loop nulls the
An on-chip one-shot determines the pulse width of the
pre-equalized transmit signal. This requires an external
capacitor connected to pins TxCAP0 and TxCAP1. The
proper value for this one-shot is 680pF. Pre-equalization
can be disabled by shorting TxCAP0 and TxCAP1 together.
offset voltage, forcing V to be zero. Although the
OS
capacitor on V is non-critical, the pole it creates can
DC
effect the stability of the feedback loop. To avoid stability
problems, the value of this capacitor should be at least 10
times larger than the input coupling capacitors.
The transmitter enters the idle state when it detects start
of idle on OPINP and OPINN input pins. The transmitter
maintains a minimum differential output voltage of at
least 450mV for 250ns after the last low to high transition.
The driver differential output voltage will then be within
50mV of 0V within 45 bit times.
The comparator is a high-speed differential zero crossing
detector that slices and accurately digitizes the receive
signal. The output of the comparator is fed into the
receive squelch circuit.
OP SQUELCH
The input to the optical receiver comes from a fiber optic
pre-amp. At the start of packet reception no more than
2.7 bits are received from the fiber cable and not
transmitted onto the TP outputs. The receive squelch will
reject frequencies lower than 2.51MHz.
11
MICRO-LINEAR [ MICRO LINEAR CORPORATION ]
