888-2408-002
4-13
WARNING: Disconnect primary power prior to servicing.
Overall System Theory
4.3.4.4.1 Isolation Boards
Each Z Plane has an associated Isolation Board which contains the isolation
resistors and disconnect switches for the two 4-way wilkinson combiners and the
isolation resistor for the 2-way Wilkinson combiner. The isolation resistors for the
four way combiners, R1-R8, are each 50 ohm. When all modules are working
properly, these resistors dissipate little or no power. However, when a PA fails
approximately half of the power of one module will be across the isolation resistor.
The resistors will handle the imbalance of a failed module for about 20 seconds.
Therefore when a module has failed, the isolation resistor will start to heat up. This
rise in temperature is detected by a thermistor, RT1-RT8, which is physically
attached to the resistor. The thermistor is monitored by the associated PA Controller
Board (there are two). The PA Controller then shuts off the module RF output via
the PA gate voltage and activates the associated RF switch on the isolation board.
The switch disconnects the isolation resistor from the combiner and grounds output
of the PA (E1-E8). The entire Z Plane will be operating with a slight impedance
mismatch (it is now a 3-way combiner instead of a 4-way) but is well within the
ratings of the modules. The effects are minimized by the 2 way combiner and its
isolation resistor, which will tend to absorb imbalances, and by the characteristics of
the 3dB hybrid combiner, which tends to ignore minor amplitude and phase
imbalances in the two Z Plane outputs.
4.3.4.5 3 dB Hybrid Combiner
The 3dB combiner is located in the middle of the Power Block, between the two Z
Planes. The two outputs from the Z Planes are summed in the 3dB output combiner.
Since the RF inputs to the two Z Planes, driving the PA inputs, come from the 3dB
hybrid splitter on the IPA Backplane Board, they differ in phase by 90 degrees.
Thus, the outputs from the Z Planes differ by 90 degrees, allowing them to be
combined in the 3dB hybrid output port with very little power appearing at the
reject load.
If the two Z Plane outputs are not equal, or are not exactly 90 degrees apart in phase,
a portion of the power appears to the reject load, however the 3dB hybrid is a very
stable device which sends most of the input power to the combined output over a
fairly wide range of input power ratios and input phase differences. The reject load
is a 50 ohm 800 watt resistor mounted on a heatsink which is located on the upper
end of the hybrid, just above the Z-Planes. The output of the 3dB hybrid is on the
lower end of the hybrid and is connected to the transmission line Harmonic Filter in
the rear of the cabinet, via the aluminum enclosure just above the power supply.
There is a static drain choke located inside this box.
Summary of Contents for Z5 CD
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Page 192: ...6 40 888 2408 002 WARNING Disconnect primary power prior to servicing Troubleshooting...
Page 194: ...7 2 888 2408 002 WARNING Disconnect primary power prior to servicing Parts List...
