Antenna Switch - Motorola ASTRO XTS 5000 Detailed Service Manual

3-10
3.1.3.3 Power Amplifier Transistor Q107
The power amplifier transistor, Q107, is an LDMOS FET housed in a high-power, surface-mount, ring
package. To prevent thermal damage, it is essential that the heat sink of the power module be held in
place against the radio chassis. The input impedance-matching network uses discrete inductors and
capacitors. The low-pass output matching network uses both transmission lines and lumped LCs.
Drain bias is applied through E101 and L101. Gain is dynamically controlled by adjusting the gate
bias. The gate is insulated from the drain and source so that gate bias current is essentially zero.
VHF: The input and output impedance-matching networks consist of L112–L114 and C137–C140.
Gate bias is applied through R105, R106, and L108.
UHF Range 2: The input and output impedance-matching networks consist of L113, L114 and C137,
C139, C140, C149, and C151.
Gate bias is applied through R105, R106, and L720.
UHF Range 1 and 700–800 MHz: The input impedance-matching network is L106, L107, C108, and
C109. A transmission-line structure and C137, C111, and C112 form the output-matching network.
Gate bias applied through R106 and L110.
3.1.3.4 Directional Coupler
A directional coupler senses the transmitter forward and reverse power as control signals in the
transmitter's automatic level control (ALC) loop. Isolated ports are terminated with external resistors.
VHF and UHF Range 1: The directional coupler consists of three embedded transmission lines.
UHF Range 1 and 700–800 MHz: The directional coupler is U101, a low-loss, bidirectional coupler.

3.1.3.5 Antenna Switch

NOTE: Refer to
listing of schematics that will aid in the following discussion.
The antenna switch is a single-pole, double-throw, positive-intrinsic-negative (PIN) diode, transmit/
receive (T/R) antenna switch. Forward DC bias turn the diodes ON, reverse or zero bias turns them
OFF.
VHF and UHF Range 2: PIN diodes D707 and D717 form a narrow-band, quarter-wave, T/R switch.
In transmit mode, both diodes are forward-biased, and the signal goes from the transmitter to the
antenna through the low resistance of the series PIN diode, while the low resistance of the shunt
diode shorts the receiver. In receive mode, neither diode is biased and both behave as small-value
capacitors, creating a high blocking impedance, in effect disconnecting the transmitter circuitry from
the antenna.
UHF Range 1: Diodes D701 and D702 form a broadband switch. Inductors L706 and L707 resonate
with the OFF diode parasitic capacitance to improve isolation.
700–800 MHz: PIN diodes D701 and D702 form a narrow-band, quarter-wave, T/R switch. When the
PIN diodes are OFF, parallel inductors L706 and L707 resonate with the diode parasitic capacitance
to increase isolation at the signal frequency.
The switch control circuit consists of transistors Q101, Q103, Q106, and associated resistors. The
input signals are TXINH from the VOCON board and RXH (RX for VHF) from the PCIC. When
TXINH is low and RXH (RX for VHF) is high, the switch is in the receive state. When TXINH and
RXH (RX for VHF) are low, the switch is in its transmitter state. When TXINH is held high, the radio is
inhibited from transmitting. This is a secure-module control feature.
July 21, 2004
Table 8-1, "List of Transceiver Schematics and Board Overlays," on page 8-1
Theory of Operation: Transceiver Board
6881094C31-D
for a