94 9 November 2001 Transmitters
AGC AND PROOFING WITH CW CARRIERS
Do not use AGC when proofing transmitters that operate
with CW carriers. When CW carriers are used, the AGC
circuit reduces gain by about 2.0 dB, which reduces a link’s
C/N by 2.0 dB from what would be achieved with modulated
carriers.
A coupler directs a portion of the amplified signals to an
automatic gain control (AGC) broadband detector. The DC
output of the AGC detector is routed to a comparator,
which checks the voltage against a factory-set reference. This
reference is set for modulated carriers; when constant wave
(CW) carriers are used in the AGC mode, the levels of the
carriers are 2.0 dB lower than they are when modulated
carriers are used. The DC output of the comparitor adjusts
the variable PIN attenuator.
Optical Signals Laser: The laser converts the RF signals to optical signals that
exit the module through an optical connector on the front
panel. (Output power varies by model.)
Control Signals A sample of the optical signal enters a closed loop circuit that
monitors the laser’s output power and temperature.
Photodetector: A photodetector captures this sample and
converts it to a DC voltage representing the optical output
power. Then, this voltage is sent to various monitoring
circuits.
• An optical power monitor circuit compares the voltage to a
factory-set reference level and generates a control signal.
• A bias control circuit receives information from the monitor
circuit and uses it to adjust the optical output power of the
laser, maintaining a constant power level. This control
circuit also receives the Automatic Shutdown signal, from a
2RRX return receiver, FRX forward receiver, or a remote
monitoring source. If triggered, the Automatic Shutdown
signal breaks the closed loop, disabling the laser.
• The control circuit also receives signals from the safety
interlock. If the shield protecting the optical connection is
removed, the safety interlock breaks the closed loop,
disabling the transmitter.
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