Innovator CU10BT-CU5000BT Analog Transmitter
Board Descriptions
Instruction Manual, Rev. 0
50
that is used to mute the PIN attenuator when there is no input signal. The output of the
PIN attenuator is sent to two cascaded amplifiers, U2 and U3, which are capable of
gene10 dBm average power from the board at J2.
The PIN attenuator is driven by an ALC circuit or by a manual fixed voltage bias,
depending on the position of switch S1. When the switch is pointing to the left, looking
from the front of the Drawer, the ALC circuit is enabled. When the switch is pointing to
the right, the ALC circuit is disabled and the PIN attenuator is controlled through the
Manual gain pot R62. When the switch is in either ALC or manual, the voltage in the
unused circuit is preset low by the circuitry connected to pins 4-6 on SW1. This allows
the RF power to ramp up slowly to full power when the switch changes positions. CR8,
C33 and associated components control the ramp up speed of the manual gain circuit.
CR9, C42 and their associated circuits do the same thing for the ALC circuit. The
practical effect of this is to preset the RF drive power to near zero output power when
enabling and disabling the ALC, followed by a slow controlled ramp up of power.
The ALC circuit normally attempts to hold the Drawer output power constant, but there
are four faults that can override this. These faults are Input Fault, VSWR Cutback Fault,
VSWR Shutdown Fault and Overdrive Fault.
The Input Fault is generated by comparator U7C and presets the PIN attenuator and ALC
circuit to maximum attenuation whenever the input signal drops below about -7 dBm.
Test point TP2 allows the user to measure the detected input voltage.
The VSWR cutback circuit is set so that the ALC circuit will start reducing RF drive once
the Reflected power reaches a level of about 6% and will keep reducing the drive to
maintain that level. U8A, U8B and their associated components, diode-or the metering
voltages, which generates this cutback. The forward power is scaled to 2V = 100 % and
the reflected power is scaled to 2V = 25%. The Reflected metering voltage is doubled
again by U8B so that when the voltage of U8B exceeds the voltage at the output of U8A,
the reflected power takes over the ALC circuit. Once the U8B voltage drops below the
forward power at U8A, the forward power takes over again.
The VSWR shutdown circuit will shut the Drawer down if the Reflected power increases
to 15% or higher, which can happen if the Drawer sees reflected power when the ALC is
in manual.
The Overdrive protection looks at a sample of the RF signal that is applied to J1 of the
board. The peak level of this signal is detected and can be measured on TP1. This
voltage is applied to a comparator with the threshold set by R38. If this threshold is
exceeded, the ALC circuit mutes then ramps up to try again. This circuit also works in
manual gain as well.
