Receive Section
The current output of photodiode detector CR2 is converted to a voltage using U9, which is configured as a
switchable gain transimpedance amplifier. Amplifier U9 is a low noise, high speed operatonal amplifier. The
normal gain is controlled by R28. For large input signals, the gain is reduced to avoid saturation of the amplifier
output. This is accomplished by swiching in shunt resistor R27 across the normal feedback resistor R28 using
analog switch U8 (pins 14 - 15). The switch side of R27 (U8 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 C25 and switch U8 pins 6 - 7. Switch section U8
pins 10 - 11 is used as a logic inverter to generate complementary signals for the various switch sections. Both
LC and RC filtering is used on the power supplies of the amplifier in order to reduce noise pickup.
Detector CR2 is connected to the +15 volt source through an RC filter formed by R25 and C16. 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 R25. The voltage at pin 2 of CR2 therefore decreases from +15 volts proportionally with
the average light input, reaching near 0 volts with an average light level of 0 dBm. Amplifier U10A and associ-
ated circuitry form a differential amplifier that produces an output of approximately 0 to +10 volts for input light
levels of no light to 0 dBm respectively. The second section of U10 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 U9. Resistor R38 provides hysteresis for the
comparator.
The output of U9 drives two differential amplifier sections consisting of U11 and U12. The stages are AC cou-
pled by capacitors C30, C38, and C39, which act as high pass filters. High frequency gain is limited by capaci-
tors C40 and C45. This reduces the noise bandwidth, allowing better receiver sensitivity. The gain is set by re-
sistors R41, R42, R44 and R45. These amplifiers provide limiting for large input signals. Both LC and RC filter-
ing are again used on the power inputs of these amplifiers.
Capacitor C44 couples the signal at U12 pin 1 to U13A, which performs as a linear amplifier with a gain of 10 for
small signals. This stage also provides limiting for large signals. The signal then passes through a Schmitt trig-
ger circuit formed by two sections of U13 and resistors R46 and R47. The output of this stage is the digital Rx
Data signal. The other three sections of U13 are not used.
RFL 9300 RFL Electronics Inc.
January 19, 2005 18 - 12 (973) 334-3100
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