Transmitter Section
Receiver Section
The transmitter section includes the transmitter optical
subassembly (TOSA) and laser driver circuitry. The TOSA,
containing an 850 nm VCSEL (Vertical Cavity Surface Emit-
ting Laser) light source, is located at the optical interface
and mates with the LC optical connector. The TOSA is
driven by a custom silicon IC, which converts differential
logic signals into an analog laser diode drive current. This
TX driver circuit regulates the optical power at a constant
level provided the data pattern is valid 8B/10B DC bal-
anced code.
The receiver section includes the receiver optical subas-
sembly (ROSA) and amplification/quantization circuitry.
The ROSA, containing a PIN photodiode and custom tran-
simpedance preamplifier, is located at the optical inter-
face and mates with the LC optical connector. The ROSA is
mated to a custom IC that provides post-amplification and
quantization. This circuit also includes a Signal Detect (SD)
circuit which provides an LVTTLcompatible logic low out-
put in the absence of a usable input optical signal level.
Signal Detect
TX Disable
The Signal Detect (SD) output indicates if the optical input
signal to the receiver does not meet the minimum detect-
able level for Fibre Channel compliant signals. When SD is
low it indicates loss of signal. When SD is high it indicates
normal operation. The Signal Detect thresholds are set to
indicate a definite optical fault has occurred (e.g., discon-
nected or broken fiber connection to receiver, failed trans-
mitter).
The HFBR-5921L/5923L accepts a transmit disable con-
trol 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 (e.g., eye
safety circuit activated), cycling this control signal resets
the module. The TX Disable control should be actuated
upon initialization of the module. See Figure 6 for product
timing diagrams.
Functional Data I/O
TX Fault (Available only on the 2 x 6)
Avago’s HFBR-5921L/5923L fiber-optic transceiver is de-
signed to accept industry standard differential signals. In
order to reduce the number of passive components re-
quired on the customer’s board, Avago has included the
functionality of the transmitter bias resistors and coupling
capacitors within the fiber optic module. The transceiver
is compatible with an “AC-coupled” configuration and is
internally terminated. Figure 1 depicts the functional dia-
gram of the HFBR- 5921/5923.
The HFBR-5923L 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 tran-
sient fault can be cleared by cycling the TX Disable control
input.
Caution should be taken to account for the proper inter-
connection between the supporting Physical Layer inte-
grated circuits and the HFBR-5921L/5923L . Figure 4 illus-
trates the recommended interface circuit.
Eye Safety Circuit
For an optical transmitter device to be eye-safe in the
event of a single fault failure, the transmitter will either
maintain normal, eye-safe operation or be disabled. In the
event of an eye safety fault, the VCSEL will be disabled.
1.3
1.0
0.8
0.5
0.2
0
–0.2
0
x1
0.4
0.6 1-x1 1.0
NORMALIZED TIME (IN UI)
Figure 3. Transmitter eye mask diagram and typical transmitter eye.
3
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