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10. Explanation of Major Circuits
Four major circuit blocks are explained in their basic form and using signal flows.
10-1
Main DC Power Supply
10-2
Power Amp/ L.P.F.
10-3
RF Power Detector/ TX-RX Switching
10-4
Control Board
10-1
Main DC Power Supply
The main DC power supply feeds the 42V DC power to the final PA stage. It is a
state of art, switching power supply with high efficiency. A part of 42V line is
converted to 24V using a DC-DC converter and is supplied to the cooling fan.
10-2
Power Amp (PA PC1662V) / L.P.F. (PC1817)
The RF PA is the heart of this amplifier and is composed of four VRF150 MOS
FET's made by Microsemi of USA. The amplifier is a parallel push-pull, class AB
amplifier. The gate bias supply circuit is regulated for the best stability and is
thermally compensated.
The PA has a 6dB attenuator on the input for gain reduction and to enhance the
stability of the wide band power PA. The heat sink has two different thermal
sensors to detect temperatures of 50 deg.C, and 80 deg.C, respectively. At 50 deg.C,
the cooling fan shifts into high-speed mode. At 80 deg.C, the amplifier will lock the
amplifier into RECEIVE mode to protect the power FET's.
In the L.P.F. (output low pass filter) circuit, there are seven band filters that are
selected by the manual band switch. Each L.P.F. is used to reject harmonics so that
the amplifier meets international telecommunication equipment standards,
including FCC rule.
10-3
RF Power Detector / TX-RX Switch (PC1821)
As illustrated in the block diagram, there are two RF power detectors on this
board. One detects the drive signal level from the radio and the other monitors the
outgoing power and the reflected power from the load (antenna).
The TX-RX switch (Send-Receive switching) will switch the flow of drive
power/output power and the incoming signal from the antenna with two
high-speed relays mounted at the input and output sides of the PA.
14
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