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 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.
Receive Section
The avalanche photodiode (APD) CR9 is the detector for incoming light signals. CR9 is operated in the reverse
breakdown region with its dc reverse current reguated to achieve maximum sensitivity. Current regulation yields
a flat photomultplcation factor over the operating temperature range.
The high voltage for the reverse breakdown mode is generated from the LC tank circuit formed by L4 (180uH)
and C18-21 (4 - 470 pF capacitors in series) and resonated by switching transistor Q3 and C25 and C26. R22
and C22 form a voltage doubler. The high voltage sine wave is rectified and filtered by CR2-CR7 and R23, 24
and C27, 28. The tank circuit oscillates at 1.1 Mhz.
A shunt current regulator is used to keep a constant dc reverse bias current through CR9. The regulator is
formed by operational amplifier U5, transistor Q4, and their associated components. The dc voltage across R31
reflects the reverse bias current through CR9. This is compared to a reference voltage set by potentiometer R9
at the input to U5, which drives transistor Q4 (shunting element) that in turn steers excess current away from
CR9 in order to maintain a constant dc reverse bias current in CR9. By adjusting R31 to vary the reverse bias
current, the optimum point in the avalanche region can be located for each APD.
The signal from CR9 in response to the incident light signal is coupled by C33 to U6 (low noise amplifier). The
output of U6 drives two differential amplifier sections consisting of U7 and U8. The stages are ac coupled by
capacitors C35, C41 and C42, which act as high pass filters. High frequency gain is limited by capacitors C40
and C51. This reduces the noise bandwidth, allowing better receiver sensitivity. The gain is set by resistors R38,
R39, R41 and R42. These amplifiers will limit symmetrically for large input signals. Both LC and RC filtering are
again used on the power inputs to these amplifiers.
Capacitor C50 couples the signal at U8 pin 8 to U9F, which performs as a linear amplifier with a gain of 10 for
small signals. This stage will also provide limiting for large signals. The signal then passes through a schmitt
trigger circuit formed by two sections of U9 and resistors R45 and R46. The output of this stage is the digital Rx
Data signal. The other three sections of U9 are not used.
18.5.2 RFL 93 FT/FR-13M and RFL 93 FT/FR-13S
Overview
The RFL 93 FT/FR-13M and RFL 93 FT/FR-13S single width fiber optic I/O modules contain both LED emitter
and photodiode detector circuitry. The transmit section consists of the LED, LED current driver, and alarm cir-
cuits to detect misoperation due to component failures. The receive section consists of the fiber optic photode-
tector, a switched gain transimpedance amplifier, several stages of amplification and a schmitt trigger limiter. A
block diagram is shown in Figure 18-6, and a schematic diagram is shown in Figure 18-11.
RFL 9300 RFL Electronics Inc.
January 19, 2005 18 - 8 (973) 334-3100
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