Transmitter Detailed Functional Description - Motorola GTX/LCS 2000 Service Manual

Transmitter Detailed
Functional Description
7-4 Theory of Operation
• Second IF amplifiers
• FM Demodulator
• Audio amplifier
In addition to BBR IC, the back end contains the following components:
• Second LO resonator for 44.645 MHz (Y5211)
• Crystal filters for 455 kHz (FL5201, FL5202)
• Demodulator filter (C5224)
The IF signal routed to BBR IC is amplified, converted to 455 kHz, filtered,
limited and demodulated.
Demodulated audio comes out of BBR IC at pin 28 and is fed to the ASFIC, which
is part of the radio controller section.
In addition to the audio output signal, the receiver section provides an RSSI
(Receiver Signal Strength Indicator) at pin 11. The RSSI signal is fed to the
controller.
ADAPT signal in pin 22 controls the BBR IC. The normally low (0 V) ADAPT
signal is high (5 V) during change of radio channels or at turn-on. The ADAPT
signal is used to control precharge of capacitor C5231 at DEMOD_OUT. This is
done in order to eliminate the transient during a change in the frequency or when
the radio goes from transmit to receive mode.
The 15-W PA is a three-stage amplifier used to amplify the output from the
injection string to the radio transmit level. It consists of driver stage Q6501,
followed by two-stage Class C Power Module U6501.
The two stages of the power module operate from the A+ supply voltage. The
module is switched on/off by the K9.1 line through switch U6502 (which connects
the A+ line to pin 2 of U6501 when transmit is enabled).The RF drive, which is
routed into transistor Q6501, is controlled from Q6506 via the PA control line. The
rising control voltage on the PA control line causes a rising collector voltage on
Q6501. This causes more power out of the stage. Conversely, a decreasing control
line voltage decreases the power delivered into the next stage. By controlling the
drive power to U6501 and the following stages in the power amplifier lineup, ALC
loop is able to regulate the output power of the transmitter.
The antenna switch is switched synchronously with the keyed 9.1 voltage. In the
transmit mode, this 9.1 voltage is high—turning on diodes CR6502, CR6503 and
CR6504. When CR6502 is turned on, it forms a low impedance to the RF transmit
path and allows the signal to pass through. Diodes CR6503 and CR6504 short the
ends of quarter wavelength lines to ground, presenting a high impedance towards
the receiver. In this way no power is delivered into the receiver, the transmit path
remains undisturbed, and the receiver is protected during transmission.
In the receive mode, all these diodes are off. The off capacitance of CR6502 is
tuned by L6508 to form a high impedance looking into the transmitter. Therefore,
energy coming in the receive mode is channeled to the RX port. Harmonics of the
transmitter are attenuated by the harmonic filter. The harmonic filter is formed by
inductors L6513 and L6518 as well as capacitors C6540-C6542, C6551-C6553.