User's Manual
UHF BI-DIRECTIONAL AMPLIFIER TECHNICAL MANUAL
9
VGA and PA output port termination
Due to the ALC loop, the VGA maximum gain can be as high as 120dB. If the VGA output is not
connected, (or terminated by a highly reflected load), the last stage of amplification (i.e. the PA driver)
can be damaged. In such a case, the VGA still works but it won’t be able to drive the PA unit to the
nominal output power. The operator should pay attention to the diplexer and channel filter connection,
since an incorrect channel filter may reflect the output power backward and hence cause VGA damage.
The PA unit does have an output protection circuit. If more than +26dBm is detected as returned power
on the output port, the PA gain and output power will be reduced, and the ALM red LED will turn ON.
In order to bring the PA back to its normal operation state, the operator should turn off the power
supply and verify the PA output connection before turn on the unit again.
IT IS RECOMMENDED THAT THE VGA AND PA SHOULDN’T BE TURNED ON IF THEIR
OUTPUT PORTS ARE NOT CORRECTLY LOADED.
Visual Interface LEDs
The VGA has 4 visual interface LEDs: two for the biasing voltage status and two for the optimum input
power level adjustment.
The PA has 4 visual interface LEDs: two for the biasing voltage status and two (working in Exclusive
OR logic) indicating quality of the PA output connection.
Table III: LED description
Location LEDname Designation Description Color
VGAUnit +5V_B D1A ON=the+5V_BispresentintheVGA Green
VGAUnit +5V_A D1B ON=the+5V_AispresentintheVGA Green
VGAUnit P_INLO D2A
ON=theinputpowerislowerthanorequal
tothedesiredlevel Green
VGAUnit P_INHI D2B
ON=theinputpowerishigherthanorequal
tothedesiredlevel Green
PAUnit +5_PA D5A ON=the+5Varepresents inthePAsection Green
PAUnit +48V_PA D5B ON=the+52VispresentinthePAsection Green
PAUnit PA_OUT_SWR
D6A
D6B
GreenLEDON=thereflectedpowerislower
than26dBm.PAatnormalbiasing.
Green
or
Red
RedLEDON=ifthereflectedpowerisequal
toorhigherthan+26dBm.PAstillworking
butatreducedoutputpower.