Uhf Transmitter; Power Amplifier (Pa); Antenna Switch - Motorola PRO3150 Service Manual

UHF Transmitter

2.5 UHF Transmitter
The UHF transmitter consists of the following basic circuits as shown in Figure 2-4.
• Power amplifier (PA)
• Antenna switch/harmonic filter
• Antenna matching network
• Power control integrated circuit (PCIC)
From VCO
2.5.1 Power Amplifier (PA)
The PA consists of two LDMOS devices:
1. PA driver, U101
2. PA final stage, Q110
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 (U101-6 and -7) with an input signal of 2mW at 3dBm
at U101-16. The current drain is typically 160mA while operating in the frequency range of 403-
470MHz (Band 1) or 450-527 MHz (Band 2).
The LDMOS PA is capable of supplying an output power of 7W with an input signal of 0.3W. The
current drain is typically 1300mA while operating in the frequency range of 403-470MHz. The power
output can be varied by changing the bias voltage.

2.5.2 Antenna Switch

The antenna switch circuit consists of two PIN diodes (CR101 and CR102), a pi network (C107, L104
and C106) and two current limiting resistors (R101 and R170). In the transmit mode, B+ at PCIC
(U102) pin 23 goes low turning on Q111 which applies a B+ bias to the antenna switch circuit to bias
the diodes "on". The shunt diode (CR102) shorts out the receiver port and the pi network. This
operates as a quarter wave transmission line to transform 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
Vcontrol
PA
Driver
Figure 2-4. UHF Transmitter Block Diagram
Vcontrol

Antenna Switch/

PA-Final
Matching Network/
Harmonic Filter
Stage
Antenna
Jack
2-5