Low Band Transmitter; Power Amplifier (Pa); Figure 2-15:Low Band Transmitter Block Diagram; Antenna Switch - Motorola Professional Radio Service Manual

2.13 Low Band Transmitter

The low band transmitter consists of the following basic circuits as shown in Figure 2-15.
•

Power amplifier (PA).

• Antenna switch/harmonic filter.
• Antenna matching network.
• Power control integrated circuit (PCIC).
From VCO
2.13.1 Power Amplifier (PA)
The PA consists of two LDMOS devices:
• PA driver, U101.
• PA final stage, Q100.
The LDMOS driver (U101) provides 2-stage amplification using a supply voltage of 7.3V. The
amplifier is capable of supplying an output power of 0.3W (pins 6 and 7) with an input signal of 2mW
at (pin16). The current drain is typically 120mA while operating in the frequency range of 29.7 - 50
MHz. The power output of this stage is varied by the power control loop which controls the voltage on
pin 1.
The LDMOS PA is capable of supplying an output power of 8W with an input signal of 0.3W. The
current drain is typically 2000 mA while operating in the frequency range of 29.7 - 50 MHz. The final
stage gate is bias by a voltage from PCIC, pin 24. This voltage is the output of a programmable DAC
inside the PCIC and the output is adjustable with the radio tuner.
2.13.2

Antenna Switch

The antenna switch circuit consists of two pin diodes (D100 and D101), a RF network (C147 and
L103), and a DC feed network (L104, C144, and current limiting resistor R101). In the transmit mode,
PCIC (U102) pin 32 goes high supplying current via the feed network to bias the diodes "on". The
shunt diode (D101) shorts out the receiver port and L103 is connected from the RF path to ground.
L103 and the input capacitance of the lowpass filter form a parallel resonant circuit, effectively
disconnecting the receiver port from the antenna while not loading the transmit path. In the receive
mode, pin 32 goes low and the diodes are off. D100 looks like a high impedance effectively
SPI Bus
Vcontrol
Power Amplifier (PA)
PA
Driver
Figure 2-15: Low Band Transmitter Block Diagram
Antenna Switch Bias
PCIC
Gate Bias
PA Final
Stage
Antenna
Matching
Network
Antenna Switch/
Harmonic Filter
2-19