Antenna Switch - Motorola PRO3150 Service Manual

2-12
U241-10 is injected into the input of the PA module (U101-16). Also in transmit mode, the audio signal
to be frequency modulated onto the carrier is received through U201-41.
When a high impedance is applied to U241-19, the VCO operates in battery saver mode. In this
mode, both the receive and transmit oscillators, as well as the receive transmit and prescaler buffer,
are turned off.
2.9 VHF Transmitter
The VHF transmitter consists of the following basic circuits as shown in Figure 2-9:
•Power amplifier.
•Antenna switch/harmonic filter.
•Antenna matching network.
•Power control integrated circuit (PCIC).
From VCO
2.9.1 Power Amplifier
The power amplifier consists of two devices:
•LDMOS driver IC (U3501) and
•LDMOS PA (Q3501)
The LDMOS driver IC contains a 2-stage amplification with a supply voltage of 7.3V.
This RF power amplifier is capable of supplying an output power of 0.3W (pins 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 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 can be 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 three current limiting resistors (R3571, R3572, R3573). In the transmit mode,
B+ at PCIC (U3502) pin 23 will go low and turn on Q3561 where a B+ bias is applied 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), and transforms the low impedance of
PCIC
Vcontrol
PA
Driver
Figure 2-9. VHF Transmitter Block Diagram
Vcontrol
Antenna Switch/
PA-Final
Matching Network/
Stage
Harmonic Filter
VHF Transmitter
Antenna
Jack