DE
DANIELS
ELECTRONICS
UHF Amplifier Instruction Manual UT-3 406 - 512 MHz
2-3
2.4
Power Control Circuitry
The UT-3/400 Amplifier employs a closed loop power control which uses a sample of the forward
RF power to control the gain of RF amplifier U4. Op-amp U1b compares the sampled RF voltage
to the output power setpoint and generates an error signal which Q3 uses to control the voltage on
U4's gain control pin. The output power setpoint is determined by R7, the Output Power Adjust
potentiometer.
In low power amplifiers jumper JU5 is installed and the forward power is sampled by TL3, D6,
C27, R18, L11, R19, and C28. In high power amplifiers jumper JU5 is not installed and the
forward power is sampled by TL1, D5, C14, R13, L6, R12, and C15. In both high power and
low power amplifiers, the power control circuitry keeps the output power of U4 constant.
Therefore, in high power amplifiers, the amplifier's output power will fluctuate with variations in
the +13.8 Vdc supply voltage.
2.5
Power Sensing Circuitry
The UT-3/400 Amplifier is equipped with output power and VSWR sensing lines which can be
individually configured as open collector or linear outputs. In open collector configuration, the
output is active low, that is, when a fail condition is detected (not enough output power or too high
antenna VSWR) the open collector transistor is turned on. In linear configuration, a voltage
proportional to the sensed output power or antenna VSWR is output.
Both the Output Power Alarm setpoint and the VSWR Alarm setpoint are individually adjustable;
however, the Output Power Alarm setpoint must always be adjusted before the VSWR Alarm
setpoint. This is because the Output Power Alarm setpoint is used as a reference by the VSWR
Alarm circuitry.
2.5.1 Output Power Sense
The output power sense circuitry uses directional coupler TL3 to sample some of the forward
power. The sampled power is rectified by diode D6 and capacitor C27 and then amplified by
op-amp U3b. Op-amp U3b's amplification is controlled by R21, the output power alarm adjust
potentiometer. The amplified voltage is then output directly in linear operation (JU1 installed, JU2
not installed) or compared to +6.3 Vdc by op-amp U1a which then drives transistors Q5 and Q6
for open collector operation (JU1 not installed, JU2 installed).
In open collector configuration, Q6 (the open collector output transistor) is turned on when an
alarm condition occurs. The adjustment range for the output power alarm can vary depending on
the setting of R21 (the Output Power Alarm Adjust potentiometer).
