2
Functional Description
Receiver Section
Design
The receiver section for the
HFCT-5750xxx contains an
InGaAs/InP photo detector and
a preamplifier mounted in an
optical subassembly. This optical
subassembly is coupled to a
postamp/decision circuit on a
circuit board.
The postamplifier is ac coupled
to the preamplifier. The
coupling capacitors are large
enough to pass the SONET/SDH
test pattern at 622 Mb/s without
significant distortion or
performance penalty. If a lower
signal rate, or a code which has
significantly more low frequency
content is used, sensitivity, jitter
and pulse distortion could be
degraded.
There is a filter function which
limits the bandwidth of the
preamp output signal. The filter
is designed to bandlimit the
preamp output noise and thus
improve the receiver sensitivity.
Loss of Signal
The Loss of Signal (LOS) output
indicates that the optical input
signal to the receiver does not
meet the minimum detectable
level for compliant signals.
When LOS is high it indicates
loss of signal. When LOS is low
it indicates normal operation.
The Loss of Signal thresholds are
set to indicate a definite optical
fault has occurred (eg.,
disconnected or broken fiber
connection to receiver, failed
transmitter).
Transmitter Section
Design
A schematic diagram for the
transceiver is shown in Figure 1.
The HFCT-5750xxx incorporates
an FP laser as its optical source.
All parts have been designed to
be compliant with IEC 825 eye
safety requirements under any
single fault condition and CDRH
under normal operating
conditions. The optical output
is controlled by a custom IC that
detects the laser output via the
monitor photodiode. This IC
provides both dc and ac current
drive to the laser to ensure
correct modulation, eye diagram
and extinction ratio over
temperature, supply voltage and
operating life.
Tx Fault
The HFCT-5750xxx module
features a transmit fault control
signal output which when high
indicates a laser transmit fault
has occurred and when low
indicates normal laser
operation. A transmitter fault
condition can be caused by
deviations from the
recommended module operating
conditions or by violation of eye
safety conditions. A fault is
cleared by cycling the Tx Disable
control input.
Tx Disable
The HFCT-5750xxx accepts a
transmit disable control signal
input which shuts down the
transmitter. A high signal
implements this function while a
low signal allows normal laser
operation. In the event of a fault
(eg., eye safety circuit
activated), cycling this control
signal resets the module. The Tx
Disable control should be
actuated upon initialization of
the module.
Figure 1. Transceiver functional diagram
HFCT-5760xx BLOCK DIAGRAM
EEPROM
ELECTRICAL INTERFACE
LOS
DATA IN
DATA IN
TX_DISABLE
TX_FAULT
TRANS-
IMPEDANCE
PRE-
AMPLIFIER
FILTER
AMPLIFIER
OUTPUT
BUFFER
DATA OUT
DATA OUT
LASER
DRIVER
PHOTODIODE LASER
BIAS
CONTROL
MODULATOR
&
SAFETY
CIRCUITRY
FP
LASER
MOD-DEF (2)
MOD-DEF (1)
MOD-DEF (0)