3
Transmit Disable (Tx_Disable)
The AFBR-59R5LZ accepts a
TTL transmit disable control
signal input which shuts down
the transmitter. A high signal
implements this function while
a low signal allows normal
transceiver operation. In the
event of a fault (e.g. eye safety
circuit activated), cycling this
control signal resets the
module as depicted in Figure
5. An internal pull down
resistor enables the laser if the
line is not connected on the
host board. Host systems
should allow a 10ms interval
between successive assertions
of this control signal.
Tx_Disable can be asserted via
the two-wire serial interface
(address A2h, byte 110, bit 6)
and monitored (address A2h,
byte 110, bit 7).
The contents of A2h, byte 110
bit 6 are logic Or’d with the
TX_DISABLE pin to control the
transmit output.
Eye Safety Circuit
The AFBR-59R5LZ provides
Class 1 (single fault tolerant)
eye safety by design and has
been tested for compliance
with the requirements listed in
Table 1. The eye safety circuit
continuously monitors optical
output power levels and will
disable the transmitter upon
detecting an unsafe condition
beyond the scope of Class 1
certification. Such unsafe
conditions can be due to
inputs from the host board
(Vcc fluctuation, unbalanced
code) or a fault within the
transceiver.
Receiver Section
The receiver section contains a
PIN photodiode and custom
transimpedance preamplifier,
located at the optical interface
which mates with the LC
optical connector. The output
is fed to a custom IC that
provides post-amplification and
quantization.
Signal Detect (Sig_Det)
The post-amplification IC also
includes the transition
detection circuitry which
monitors the ac level of
incoming optical signals and
provides a TTL status signal to
the host. An adequate optical
input results in high signal
detect output while a low
signal detect output indicates
an unusable optical input. The
signal detect thresholds are set
so that a low output indicates
a definite optical fault has
occurred. Signal Detect can be
monitored via the two-wire
serial (address A2h, byte 110,
bit 1).
Component Monitoring
The AFBR-59R5LZ real-time
monitors of Tx_Bias, Tx_Power,
Vcc, Temp and Rx Average
Power may potentially be used
as a debugging aid for system
installation and design, and
transceiver parametric
evaluation for factory or field
qualification. For example,
temperature per module can be
observed in high-density
applications to facilitate
thermal evaluation of blades
and systems.
Transmitter Section
The transmitter section
contains 850nm VCSEL
(Vertical Cavity Surface
Emitting Laser) light source,
located at the optical interface
which mates with the LC
optical connector. The VCSEL
is driven by a custom IC which
uses the incoming differential
(PECL compatible) high speed
logic signal to modulate laser
diode driver current. This Tx
laser driver circuit regulates
optical output power at a
constant level provided the
incoming data pattern is dc
balanced (8B/10B code, for
example).