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10. Explanation of Major Circuits
Six major circuit blocks are explained in their basic form and using signal flows.
Refer to the Block Diagram on the final page.
10-1
RF Power Detector / TX-RX Switch (PC1821)
10-2
Frequency Detector Unit
10-3
PA Unit
10-4
LPF Unit
10-5
Control Unit
10-6
Power Supply
10-1
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.
The drive signal level of forward power and reflected power is sent to the control
circuit.
10-2
Frequency Detector Unit
This unit detects the frequency of the drive power from the transceiver. When
frequency beyond the 26-28MHz amateur bands is detected, the amp will stop
operating.
10-3
Power Amp (PC1662V)
The RF PA is the heart of this amplifier and is composed of four VRF150 MOS
FET's made by Microsemi of USA, gaining 550W max. output. 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.
15
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