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WARNING: Disconnect primary power prior to servicing.
Section 4 Theory of Operation
Maxiva ULX COFDM Series
Operation. The GUI screen is present only in the main TCU (cabinet 1 in multi cabinet
systems).
4.4.2
Transmitter RF Power Control
The IPA and PA modules have no gain adjustment. The transmitter RF output power is
controlled via the Phase and Gain Board located in the Predriver modules. The
Predrivers are the only components in the drive chain (besides the exciter) capable of
adjusting their RF power based on a command from the TCU.
The TCU in the main cabinet contains a PCM (processor control module) which acts as
the transmitter controller. However, a depopulated TCU is present in each PA cabinet.
The TCU monitors the power sample from its cabinet and issues the proper voltage to
the Predrivers to maintain the cabinet power regulation. This ALC loop resides in each
cabinet.
There is no System ALC Loop.
In addition to providing main cabinet power regulation, the Main TCU also has the role
of issuing the cabinet power reference. This reference is used by the ALC loop in each
cabinet to raise or lower the transmitter power as commanded by the end user. This
signal is sent via CAN messages. Figure 4-2 on page 4-6 illustrates the power control
functionality. If the Main TCU is down, all cabinets maintain their current power
output.
In each cabinet, the system power reference is compared to the detected cabinet RF
power sample and the resultant error voltage (cabinet # ALC) is produced. The error
voltage is used to control the predriver gain, which determines the cabinet output
power.
The local TCU power raise and lower buttons provides a means of individual cabinet
power control with the ALC Enabled. This provides a means of setting and calibrating
the individual cabinet output power to ensure correct cabinet combining in a multiple
cabinet system.
The TCU in each cabinet has power control AUTO and Manual buttons. These buttons
turn cabinet ALC on and off. If the ALC is disabled, the cabinet power can be raised or
lowered via the local TCU power raise and lower buttons. This is an open loop mode
and is intended primarily as a cabinet test mode (cabinet operated independently of the
main TCU).
The TCU samples the cabinet forward power ten times per second. For digital TV the
ALC loop response does not need to be any faster then this. It’s main role is to maintain
cabinet power regulation to compensate for thermal drift in the amplifiers.
Summary of Contents for Maxiva ULX COFDM Series
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