4-12
888-2406-002
WARNING: Disconnect primary power prior to servicing.
Overall System Theory
4.3.4.4 Power Amplifier (PA)
The RF drive from the IPA enters the PA Amplifier Boards at Pin C. The RF input
level for each amplifier is typically 15-20 watts. TL1, C1 and C2 allow the input
impedance to be matched to 50 ohms and permit the phase shift through the module
to be trimmed to a nominal value which allows all modules to combine in-phase.
The RF signal is then sent to T1, a coaxial 9:1 transformer which also splits the
phase providing 0 degree and 180 degree outputs to drive a pair of push-pull RF
power MOSFETs (Q1 and Q2). Bias to Q1 and Q2 (labeled VG for Gate Voltage) is
connected through the center of the secondary winding of T1.
The control voltage input called VG can be thought of as a bias voltage which can
be used to vary the amplifier output. Normally for the PAs it is simply used to turn
the amplifiers on and off. If VG is -18V the amplifier is off or muted. If VG is 0V
the amplifier is fully turned on and drive level from the IPA determines the
amplifier power output.
When the amplifier is used as the IPA, VG is used to linearly vary the IPA output
which then controls the transmitter power output. For an IPA, VG comes from the
Life Support Board and is controlled by the APC loop on the Master Controller
Board. Pressing the RAISE and LOWER controls on the front of the transmitter will
vary the APC voltage which in turn will vary VG to the IPAs. This will vary the IPA
RF output, which will raise or lower transmitter power.
The MOSFET outputs are combined in T2, a 1:4 transformer as shown in Figure 4-
6. C19 and C20 fine-match the 50 ohm load impedance at the PA output to the
output impedance of T2. The +52Vdc or VD (Drain Voltage) from the main power
supply enters at location C-1 and is decoupled and filtered by C11, C12, C13, C14,
C16, C24, L1 and L2. The DC voltage reaches the drain of MOSFETs Q1 and Q2
through the center tap of T2’s primary winding. The advantage of this method is to
allow direct DC connection to the MOSFETs and have an AC RF output through the
coax without the need for a blocking capacitor.
Figure 4-6
PA Simplified Block Diagram
Summary of Contents for Platinum Z2 CD
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Page 70: ...3 14 888 2406 002 WARNING Disconnect primary power prior to servicing Operator Guide ...
Page 130: ...4 60 888 2406 002 WARNING Disconnect primary power prior to servicing Overall System Theory ...
Page 194: ...6 38 888 2406 002 WARNING Disconnect primary power prior to servicing Troubleshooting ...
Page 196: ...7 2 888 2406 002 WARNING Disconnect primary power prior to servicing Parts List ...
