Antenna Switch; Harmonic Filter - Motorola ASTRO XTSTM 4000 Detailed Service Manual

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3-8

3.1.3.3 Power Amplifier Transistor Q107

The power amplifier transistor, Q107, is an LDMOS FET housed in a high-power, surface-mount

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 lumped LCs. Drain bias is applied through

E101, R130 and L105. 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. Gate bias is applied

through R102, R105, and R106.

VHF: The input and output impedance-matching networks consist of C112–C115, C120–C124,

L103–L104, L106–L108.

UHF: The input and output impedance-matching networks consist of C114, C120–C122, C124,

C141, L108 and L113.

3.1.3.4 Antenna Switch

NOTE: Refer to

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 (via Q104) turn the diodes ON, reverse or zero bias

turns them OFF.

VHF: PIN diodes D707 and D717 form a narrow-band, quarter-wave, T/R switch.

UHF: PIN diodes D701 and D702 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.

The harmonic filter is a high-power, low-loss, low-pass filter. Its purpose is to suppress transmitter

harmonics. The filter also improves receiver out-of-band rejection. Shield SH07 must be in place to

achieve the required stop band rejection. The harmonic filter uses discrete components.

VHF: The pass band is up to 190 MHz, and the stop band is above 260 MHz.

UHF: The pass band is up to 490 MHz, and the stop band is above 760 MHz.

May 28, 2008

Table 8.1, "List of Schematics and Boards Overlays," on page 8-1

Theory of Operation: Transceiver Section

for a listing of

6871620L01-C

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