Receive Section
The current output of photodiode detector CR6 is converted to a voltage using U5, which is configured as a
switchable gain transimpedance amplifier. Amplifier U5 is a low noise, high speed operational amplifier. The
normal gain is controlled by R22. For large input signals, the gain is reduced to avoid saturation of the amplifier
output. This is accomplished by switching in shunt resistor R21 across the normal feedback resitor R22 using
analog switch U6 (pins 14 - 15). The switch side of R21 (U6 pin 15) is shunted to ground when using the high
gain configuration to reduce feedback signals resulting from the leakage capacitance across the open switch.
Frequency compensation for the low gain setting is provided by C23 and switch U6 pins 10 - 11. Switch section
U6 pins 6 - 7 is used as a logic inverter to generate complementary signals for the various switch settings. Both
RC and LC filtering is used on the power supplies to the amplifier in order to reduce noise pickup.
Detector CR6 is connected to the +15 volt source through an RC filter formed by R20 and C15. The photodiode
acts as a current source, where the current is proportional to the light input. This current causes a voltage drop
across the filter resistor R20. The voltage at pin 2 of CR6 therefore decreases from +15 volts proportionally with
the averge light input, reaching near 0 volts with an average light level of 0 dBm. Amplifier U7A and associated
circuitry form a differential amplifier that produces an output of approximately 0 to +10 volts for input light levels
ranging from no light to 0dBm respectively. The second section of U7 is configured as a comparator, where the
output (pin 7) switches from negative to positive as the input light level increases past approximately -13 dBm.
This output is used to control the switches that change the gain of U5. Resistor R33 provides hysteresis for the
comparator.
The output of U5 drives two differential amplifier sections consisting of U8 and U9. The stages are ac coupled
by capacitors C31, C38, and C39, which act as high pass filters. High frequency gain is limited by capacitors
C40 and C42. This reduces the noise bandwidth, allowing better receiver sensitivity. The gain is set by resistors
R36, R37, R39 and R40. These amplifiers will limit symmetrically for large input signals. Both LC and RC fitering
are again used on the power inputs to these amplifiers.
Capacitor C45 couples the signal at U9 pin 1 to U10A, 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 U10 and resistors R43 and R44. The output of this stage is the digital
Rx Data signal. The other three sections of U10 are not used.
18.5.3 RFL 93 FT/FR-13SL and RFL 93 FT/FR-15SL
Overview
The RFL 93 FT/FR-13SL and RFL 93 FT/FR-15SL single width fiber optic I/O modules contain both the laser
diode emitter and photodiode detector circuitry. The transmit section consists of the laser diode, laser diode
control and current driver, and alarm circuits to detect misoperation due to component failures. The receive sec-
tion consists of the fiber optic photodetector, a switched gain transimpedance amplifier, several stages of ampli-
fication and a schmitt trigger limiter. A block diagram is shown in Figure 18-7 and a schematic diagram is shown
in Figure 18-13.
RFL 9300 RFL Electronics Inc.
January 19, 2005 18 - 10 (973) 334-3100
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