RX DATA TO
TRANSCEIVER
TX DATA FROM
TRANSCEIVER
TX ALM TO
TRANSCEIVER
LED
EMITTER
LIGHT OUTPUT
TO FIBER OPTIC
CABLE
LIGHT INPUT
TO FIBER OPTIC
CABLE
DETECTOR
LIGHT LEVEL
DETECTOR
TRANS-
IMPEDANCE
AMPLIFIER
GAIN
CONTROL
AMPLIFIERS
THRESHOLD
DETECTOR
DETECTOR SECTION
EMITTER SECTION
FAILURE
DETECTOR
DRIVER
Figure 18-6. Block diagram, RFL 93 FT/FR-13M (Multimode) and RFL 93 FT/FR-S (Singlemode) 1300-nm Fiber
Optic I/O Module
Transmit Section
The fiber optic LED U1 is driven from a current source generated by Q1 and Q2. The unmodulated bias current
through the LED is controlled by R2 and R45, which sets the bias voltage for emitter followers Q1 and Q2. The
voltage across R9 produces a bias current which is essentially the same as the LED current. This causes the
LED to emit a constant light output at approximately the average output power, without modulation.
Modulation is accomplished by ac coupling the transmit data signal into the current driver circuit through C6.
The signal switches the current in the LED between a maximum limited value and zero. The current is limited by
U2, which compares the voltage across R9 (at pin 6) with a reference voltage (at pin 9) formed by CR1, R4 and
R5, and limits the voltage at the base of Q1 by the current through pin 5. The maximum voltage across R9 is
about 1.2 volts, which limits the LED current to about 100 mA.
An alarm output is generated at P2 pin 2 if the voltage across the LED deviates considerably from its nominal
value due to component malfunctions. The LED voltage is filtered by R12 and C7, and is then buffered by U4A.
Voltage reference levels are generated by R10, R11, and R13. The upper reference voltage is also buffered by
U4B. Buffering is required to keep the voltages being compared from being effected by the operation of the
comparator circuitry. A window comparator is formed by dual comparator U3, whose open collector outputs are
connected to pullup resistor R19 and the Tx alarm output. Resistors R16 and R18 provide hysterisis. The alarm
output is an active low.
RFL 9300 RFL Electronics Inc.
January 19, 2005 18 - 9 (973) 334-3100
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