Transmitter; Harmonic Filter; Rf Power Amplifier Module; Alc Circuits - Motorola ASTRO XTS 3000 Service Manual

Transmitter

Harmonic Filter

RF Power Amplifier
Module

ALC Circuits

The transmitter consists of three major sections:
• Harmonic Filter

• RF Power Amplifier Module

• ALC Circuits
RF from the power amplifier (PA) module (U105) is routed through the
coupler (U104), passed through the transmit antenna switch (CR108),
and applied to a harmonic filtering network in UHF. In the case of a
VHF transceiver board, RF from the PA module (U105) is routed
through the coupler (U104), then through the harmonic filtering
network, and on to the antenna switch (CR108). The harmonic
filtering circuit is composed of the following components: L126, L127,
L128, C149, C150, and C151 (for VHF models); or L126, L127, L128,
C149, C150, and C151 (for UHF models). Resistor R128 (UHF) or R117
(VHF) provides a current-limited 5V to J2 for mobile ASTRO XTS 3000
vehicular adapter (VA) applications.
The RF power amplifier module (U105) is a wide-band, multi-stage
amplifier (three stages for the VHF models and four stages for the UHF
models). Nominal input and output impedance of U105 is 50 ohms.
The dc bias for U105 is on pins 2, 4, 5. In the transmit mode, the
voltage on U105, pins 2 and 4 (close to the B+ level) is obtained via
switching transistor Q101. Transistor Q101 receives its control base
signal as follows:
• the microcomputer keys the D/A IC to produce a ready signal at
U102 pin 3,
• the ready signal at U102 pin 3 is applied to the TX ALC IC at U101
pin 14 (5V), and
• the synthesizer sends a LOC signal to the TX ALC IC (U204 pin 40
to U101 pin 16).
When the LOC signal and the ready signal are both received, the TX
ALC IC (pin 13) sends a control signal to turn on transistor Q101.
Coupler module U104 samples the forward and reverse power of the
PA output voltage. Reverse power is present when there is other than
50 ohms impedance at the antenna port. Sampling is achieved by
coupling some of the forward and/or reverse power, and applying it to
CR102 (VHF) or CR101 (UHF) and CR103 for rectification and
summing. The resultant dc signal is then applied to the TX ALC IC
(U101, pin 2) as RFDET to be used as an RF strength indicator.
The transmit ALC circuit, built around U101, is the heart of the power
control loop. Circuits in the TX ALC module compare the signals at
U101, pins 2 and 7. The resultant signal, C BIAS, at U101, pin 4 is
applied to the base of transistor Q110. In response to the base drive,
transistor Q110 varies the dc control voltages applied to the RF PA at
U105, pin 3, thus controlling the RF power of module (U105).
Thermistor RT101 senses the temperature of the TX ALC IC. If an
abnormal operating condition exists that causes the PA slab
temperature to rise to an unacceptable level, the thermistor forces the
ALC to reduce the set power.
4-5