AdvancedA-005-002-001
The output tuning controls on a transmitter power amplifier with an adjustable PI network:
C
Answer
Antennas and transmission lines
Type
A
are involved with frequency multiplication in the previous stage
B
allow switching to different antennas
C
allow efficient transfer of power to the antenna
D
reduce the possibility of cross-modulation in adjunct receivers
Answer Notes
An adjustable PI network at the output of a transmitter's power amplifier serves as an impedance matching circuit. By adjusting the capacitors in this network (usually labeled 'Tune' and 'Load'), the operator matches the high output impedance of the amplifier to the much lower impedance of the antenna system, typically 50 ohms.
When the impedance is properly matched, the amplifier transfers radio frequency power to the antenna with maximum efficiency. This prevents excessive power from being reflected back into the amplifier, which could otherwise cause overheating or tube damage.
While a properly tuned PI network also helps attenuate harmonics, its primary function and the reason it has adjustable controls is to optimize power transfer, not to switch antennas or handle frequency multiplication.
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In an oscillator where positive feedback is provided through a capacitor in series with a crystal, that type of oscillator is a:
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The purpose of using a centre-tap return connection on the secondary of transmitting tube's filament transformer is to: