User Manual Part 1

The power amplifier (U3) is a Mitsubishi RA07M1317M module. This PA module provides

the desired RF power output level and is stable over a wide range of VSWR conditions. The PA

is driven by a SGA-6589 driver (U20) that typically provides +21 dBm output power. The PA is

turned on and off by switching the power to this driver via transistor D10.

ALC

To maintain the specified transmitter output power level, Automatic Level Control (ALC)

is used to control the drive level to the PA. The detected forward power is compared to a

reference level provided by the digital board via op amp U21A. The resulting error voltage is

applied to a power level control port of the power amplifier module. Transmitter on/off splatter

filtering is provided by an RC network (R76, R12, C43 and C141.)

T/R Switching and Harmonic Filter

The output of the power amplifier is applied to the transmit/receive RF PIN switch

(D12/D13). This is a high dynamic-range switch that is capable of passing the desired transmit

power with minimal compression. Any harmonics generated by the PA module and the RF T/R

switch are filtered by a harmonic filter that is located between the RF T/R switch and the antenna

jack.

5.2.2 VHF 20-WATT PA BOARD

NOTE: A block diagram of the PA board is shown in Figure 5-2.

Figure 5-2 VHF 20-Watt PA Board Block Diagram

5.2.2.1 VARIABLE ATTENUATOR

Two RF signals are connected to the RF module from the PA board. One is the transmit RF

output signal and the other is the receive RF input signal. The transmit signal is applied to a

variable attenuator on the PA board formed by CR4-CR7 and several other components. This

circuit adjusts the input RF level to Q6 to control the power output of the PA board. CR4-CR7 are

PIN diodes which have a very high impedance when they are reverse biased or in the off mode.