Service manual

2-12
2.9 VHF Transmitter
The VHF transmitter consists of the following basic circuits as shown in Figure 2-10.
• Power amplifier
• Antenna switch/harmonic filter
• Antenna matching network
• Power control integrated circuit (PCIC)
Figure 2-10: VHF Transmitter Block Diagram
2.9.1 Power Amplifier
The power amplifier consists of two devices:
• 9Z67 LDMOS driver IC (U3501)
• PRF1507 LDMOS PA (Q3501)
The 9Z67 LDMOS driver IC contains a 2-stage amplifier using a supply voltage of 7.3V.
This RF power amplifier is capable of supplying an output power of 0.3W (pin 6 and 7) with an input
signal of 2mW (3dBm) (pin16). The current drain is typically around 130mA while operating in the
frequency range of 136-174MHz.
The PRF1507 LDMOS PA is capable of supplying an output power of 7W with an input signal of 0.3W.
The current drain is typically around 1800mA while operating in the frequency range of 136-174MHz.
The power output is varied by changing the bias voltage.
2.9.2 Antenna Switch
The antenna switch circuit consists of two pin diodes (D3521 and D3551), a pi network (C3531,
L3551, and C3550), and two current limiting resistors (R3572 and R3573). In the transmit mode, B+
at PCIC (U3502), pin 23 goes low to turn on Q3561 applying a B+ bias to the antenna switch circuit to
bias the diodes “on”. The shunt diode (D3551) shorts out the receiver port, and the pi network, which
operates as a quarter wave transmission line, transforms the low impedance of the shunt diode to a
high impedance at the input of the harmonic filter. In the receive mode, the diodes are both off,
creating a low attenuation path between the antenna and receiver ports.
PCIC
PA
Driver
Antenna Switch/
Harmonic Filter
PA Final
Stage
Vcontrol
Vcontrol
Antenna
Matching
Network
From VCO
Power Amplifier (PA)