Instruction Manual
6X Series Liquid-Cooled Transmitter Chapter 4, Circuit Descriptions
Instruction Manual, Rev. 0 4-5
the event of a fault. The switch can be controlled by three separate sources, the
overdrive protection circuitry, the reflected power protection circuitry, and the Rabbit
microcontroller that is controlling and monitoring the amplifier tray. The overdrive
protection and reflected power protection circuitry are extremely fast, and will
remove the RF drive in a few hundred nanoseconds. Removing the RF drive will also
remove the faults, so the output of the comparators U25 and U26 that generate
these faults drive a flip flop U41, which latches the faults so the Rabbit
microcontroller is able to see them. Once the faults are recorded the Flip Flops are
reset and the drive is re-applied.
The output of the PIN switch drives three cascaded phase shifters consisting of U21,
U22 and U23 and their associated components. These three phase shifters each
have a range of about 135°, giving an overall adjustment range in excess of 360°,
allowing the amplifier to adjust the output phase of the tray to any required value.
The output of the phase shifters, RF_A, is applied to a MMIC U4 that amplifies the RF
signal to overcome any loss which occurred in the previous stages. The output of
the MMIC drives an LDMOS amplifier, Q6, which amplifies the RF output signal to an
approximate level of 1W average power for ATSC that is applied to the Directional
Coupler U31. +32 VDC needed to operate Q6 is generated by the Fan Power Supply
Board. The coupler provides a signal to the detector IC U44 which measures the
instantaneous output power of the signal. This detected level is used to drive the
overdrive detector, U25. This signal, ODVR_LVL, is not filtered so that the protection
circuitry can react quickly. The detected level is also applied to a low pass filter
consisting of R175 and C145 whose output is used for metering, RF OUT LVL.
The board has mounting, RB1, for a daughter board that contains a Rabbit
microcontroller. This microcontroller communicates with the outside world via an
Ethernet connection, and also communicates with the other microcontrollers located
in the amplifier tray using a one wire serial bus.
The various analog parameters of the board are sampled via U45 which feeds the
detected levels of each of its inputs back to the Rabbit. U45 also controls an
external multiplexer U54, which extends the number of inputs that can be sampled.
The Rabbit also controls the settings of the digital potentiometers U33-U35. These
potentiometers generate voltages used to set the Phase, U33, the Gain, U34, and the
Overdrive Fault, U35, thresholds for the amplifier tray. There is a fourth digital pot
U36 which is not used in this configuration. The U63 microcontroller, U64 buffer
amplifier and the J5 programming port are not used in this configuration.
+32 VDC and +42 VDC needed to operate the board is supplied from the Fan Power
Supply Board. The +32 VDC connects through J3-2 and the +42 VDC connects
through J3-3. There are three power supplies on the board which are current mode
flyback topology DC-DC types that use the +42 VDC as the input voltage. The power
supply using the T1 transformer produces the +5VDC labeled +5VR for use by the
rest of the board. The power supply using the T2 transformer produces the +12VDC
labeled +12V for the rest of the board. The power supply using the T3 transformer
produces the +5VDC labeled +5V to the rest of the board. There is also a 3.3V
regulator IC U52 that takes the +5V input and generates the regulated +3.3VDC
labeled +3.3V that connects to rest of the board. The Green LED DS2 is lit if the
+3.3VDC is present on the board. A sample of the +5VDC generated on the board
connects out of the board at J3-4 and is cabled to the fan power supply board for use
on that board.