Parts List

VHF Amplifier Instruction Manual VT-3 132 - 174 MHz 2-3
2.5 Power Sensing Circuitry
The VT-3/150 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 TL2 to sample some of the forward
power. The sampled power is rectified by diode D6 and capacitor C20 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 by op-amp U1a which then drives transistors Q4 and Q3 for open
collector operation (JU1 not installed, JU2 installed).
In open collector configuration, Q3 (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).
2.5.2 VSWR Sense
The VSWR sense circuitry uses directional coupler TL3 to sample some of the power reflected
from the antenna terminal. The reflected power is rectif ied by diode D7 and capacitor C25 and then
amplified by op-amp U2b. Op-amp U2b's amplification is controlled by R36, the VSWR Alarm
Adjust potentiometer. The amplified voltage is then output directly in linear operation (JU4
installed, JU3 not installed) or compared to the output power alarm setpoint by op-amp U3a which
then drives transistor Q5 for open collector operation (JU4 not installed, JU3 installed).
In open collector configuration, Q5 (the open collector output transistor) is turned on when an
alarm condition occurs. The adjustment range for the VSWR Alarm can depending on the setting
of R36 (the VSWR Alarm Adjust potentiometer).