DE
DANIELS
ELECTRONICS
2-4
UHF Amplifier Instruction Manual UT-3 406 - 512 MHz
2.5.2 VSWR Sense
The VSWR sense circuitry uses directional coupler TL4 to sample some of the power reflected
from the antenna terminal. The reflected power is rectified by diode D7 and capacitor C33 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 Q7 for open collector operation (JU4 not installed, JU3 installed).
In open collector configuration, Q7 (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).
2.5.3 VSWR Overload
The VSWR overload circuit protects the UT-3/400 Amplifier from excessive antenna VSWR by
reducing the amplifier's gain (output power) when an overload condition occurs. The VSWR
overload circuit (R38 to R41, R8, U2a, and Q4) is an extension of the VSWR sense circuit and
operates the same as the VSWR sense open collector circuit. The VSWR Overload Adjust
potentiometer (R38) reduces the voltage level of the VSWR Alarm Setpoint. The voltage set by
R38 is the compared to the output power alarm setpoint by op-amp U2a which then drives
transistor Q4. When transistor Q4 turns on, signaling an overload condition, resistor R8 is
grounded which reduces the output power setpoint. Reducing the output power setpoint lowers the
UT-3/400 Amplifier's gain and protects the amplifiers from excessive current draw resulting from
high antenna VSWR.
The VSWR overload circuit's range of adjustment depends on the setting of the VSWR Alarm
Adjust potentiometer (R36). The VSWR overload transistor Q4 can be activated at the same point
at which the VSWR alarm becomes active or the VSWR overload circuit can be disabled by turning
R38 completely counterclockwise.
