Instruction Manual

6X Series Liquid-Cooled Transmitter Chapter 4, Circuit Descriptions
Instruction Manual, Rev. 0 4-7
to amplifier pallet 5, J10 to amplifier pallet 6, J11 to amplifier pallet 7, and J12 to
amplifier pallet 8. J15 is the programming port which is initially used to program the
U4 microcontroller and is not used by the customer.
The final amplifier Pallet Slot location identification is achieved using the resistor,
each one a different value, connected to J16 for the amplifier pallet 5, J17 for the
amplifier pallet 6, J18 for the amplifier pallet 7, and J19 for the amplifier pallet 8.
+5 VDC is supplied to the bottom splitter board thru J7-3 & 1 from the Fan Power
Supply Board. The +5 VDC connects through rectifier diodes CR2 and CR3 which
drop the voltage to +3.3 VDC whose output level is maintained by C7. The +3.3VDC
is used by the ICs U3 and U4. The Green LED DS1 is lit if the +3.3VDC is present on
the board.
4.3.7 (A7 & A9-A16) 878 Amplifier Pallets, Digital Bias (1310321)
There are nine 878 Amplifier Pallets mounted in the amplifier tray. One is used as a
driver (A7) and eight are final amplifier pallets (A9-A16). Each of the 878 pallets has
approximately +17dB of gain for the UHF frequency range of 470 to 860 MHz. The
pallets have dual parallel amplifier circuits, each using a BLF878 LDMOS RF power
transistor, which operate Class AB. The driver pallet normally operates with an RF
input of .6 Watts ATSC typically produces 30 Watts ATSC output. With an RF input
of 4.5 Watts ATSC the output of the final amplifier pallet should be 225 Watts ATSC.
The paralleling network is achieved using an input hybrid splitter and an output
hybrid combiner. The RF input is applied to the hybrid splitter that produces two
outputs, one at 0° and one at -90°. Each output connects to identical amplifier
circuits. Each signal is applied to a Balun assembly that produces two 180° out of
phase outputs. The two outputs connect to a dual LDMOS transistor, configured in a
push pull arrangement, with approximately 17 dB of gain. The amplified outputs of
the transistors connect to a Balun assembly that combines the two 180° out of phase
signals into a single output. The output connects to one input of a hybrid combiner
circuit. The 0° and the -90° signals from the amplifier circuits are combined by the
hybrid combiner circuit and connected to J2 the RF output jack on the board (≈+55
dBm).
U2 and U7 are digital to analog converter ICs that generate the bias voltage for each
transistor. U1 and U5 are current sense ICs that monitor the current at the output of
the Q1 and Q2 amplifiers and provide a sample to the U6 microcontroller. R24 is a
thermistor mounted to the board, which generates a voltage reference of the
temperature of the board that is connected to the U6 microcontroller. The
microcontroller takes the parameter sample inputs and supplies a serial data output
that is buffered by U8 before it is wired to the Serial Data Line connection at J3.
The final amplifier Pallet Slot location identification for the individual amplifier pallets
is achieved by connecting the voltage at J5 to one of the resistors, each one a
different value located on the top splitter board and the bottom splitter board. Four
of the resistors are located on the top splitter board and four are located on the
bottom splitter board.
The +42 VDC needed to operate the individual pallets is supplied from the Buss Bar
Assembly thru 30 Amp fuses to the pallet at FC1. Filtering of the +42VDC is provided
by the capacitor network consisting of C24, C30, C64, C75 and C76. There are clips
containing two "spare" 30 Amp fuses located on the Fan Power Supply Board. The
fuse protected and filtered +42 VDC is applied through the .002Ω resistors R32 and