User's Manual

350-Watt Digital VHF LB Transmitter Chapter 2, System Description, Maintenance &
Remote Control Connections
DT325B, Rev. 0 2-4
is controlled by an external bias voltage
that connects to the AGC Control Board
and adjusts the AGC Pin Attenuator Bias
Voltage which is applied to the
Filter/Amplifier Board. The RF output of
the Filter/Amplifier Board is amplified by
the LB Amplifier Board and connected to
the Overdrive Protection Board. The
overdrive protection board provides a trip
point, 110%, for over power conditions
which will cut back the output power of the
Tray.
The RF Output of the Overdrive Protection
Board connects to the 3 Way Splitter
Board. The three RF outputs connect to
the three Low Band Output Amplifier
Pallets. Each amplifier pallet provides
approximately 18 dB gain. The RF signal
inputs to the Output Amplifier Boards
(+33.3 dBm) are amplified to +51.3 dBm
outputs at J2, which are connected to the
3 Way Combiner Assembly. The 3 Way
Combiner takes the three +51.3 dBm
combined inputs and combines them to
form the ˜375 Watt RF Output at J4 of the
Combiner which soldered to J2, the RF
Output Jack of the Tray.
The 3 Way Combiner Board provides a
Forward Power Sample and a Reflected
Output Power Sample that connect thru
low pass filters to the AGC Control Board.
The AGC Control Board provides detected
outputs that are used for front panel and
remote meter Indications of the forward
and reflected output power levels, AGC
Detector Voltage Level and also the VSWR
Cutback protection if the Reflected Power
level increases above the preset level.
Two voltages, +28 VDC from the internal
switching power supply and +12 VDC from
the Driver/Amplifier Assembly are needed
for operation of the Tray. The +12 VDC is
connected to a +5 VDC Regulator IC which
supplies the +5 VDC needed for operation
of the front panel mounted LEDs. The
(A10) +28 VDC Switching Power Supply
provides the +28 VDC to the Current
Metering Board. The Current Metering
Board distributes the voltages through
fuses to the Amplifier Devices on the
Filter/Amplifier, Low Band Driver Board
and the three Final Low Band Amplifier
Boards.
The Current Metering Board also supplies
sample outputs of the operating currents
of the amplifier devices in the Tray to the
front panel Current Meter. The Meter in
the (I
1
) position reads the current for the
(A3-A1) Low Band Output Amplifier
Board, (I
2
) for the (A3-A2) Low Band
Output Amplifier Board and (I
3
) for the
(A3-A3) Low Band Output Amplifier
Board. To read the desired current;
switch S2 to the proper position checking
that S1 is in the Current position. These
current readings can be used when
setting up the Idling Currents, no RF
Drive applied, for the devices. (I
1
, I
2
&
I
3
) are set for 3 Amps max.
Table 2-5: VHF Amplifier Tray Control Adjustments
ADJUSTMENT DESCRIPTION
R2 – A7
Phase
Adjusts the phase of the RF output by approximately 70°.
R3 – A6
Gain
Adjusts the gain of the RF output when the amplifier control
board is in the AGC mode.
Table 2-6: VHF Amplifier Tray Sample
SAMPLE DESCRIPTION
J5
RF Front Panel Sample
Forward power sample of the tray from the AGC control
board.