Transmitter System and PLL Synthesizer 2-3
3.0 Transmitter System and PLL Synthesizer
The transmitter consists of: Microphone, Pre-emphasis, Modulator, Drivers, Power Amplifier, Low-
Pass Filter, Antenna Switch and an Auto-Power Control.
3.1 TX Power Stage
The TX VCO output signal is channeled by the TX power stage via an attenuator. The attenuator is
pie style resister attenuator, and is used for isolation between the VCO and power amplifiers.
The TX Power stage is consist of: pre-drive (Q401), drive amplifier (Q402) and final amplifier
(Q403). The -4dBm TX RF signal from the attenuator is increased to around +25dBm by the pre-
drive and the drive amplifier.
The TX signal from drive amplifier is next amplified by final amplifier (Q403). It passes through a
strip line for impedance matching, and a low pass filter to suppress unwanted harmonics via the
antenna switch.
3.2 Antenna Switch
The antenna switch is for enabling the TX section and the RX section to share the antenna.
In transmit mode, PIN diodes (CR401, CR301, CR302) are forward biased and these enable the RF
signal to pass to the antenna.
3.3 Low Pass Filter
The TX Low Pass Filter consist of a fifth Chebyshev filter and reduces the unwanted spurious for TX
output power from final Amplifier : C426~430, L409, L410.
LPF
ANT-SW
FINAL
AMP
CURRENT
DETECT
DRIVE
AMP
APC
P.DRIVE
AMP
VCO
PLL
PLL DATA
VCTCXO
REG. 5.8V
U106
U701
Q401Q402Q403
R417
CR401
BATT.
LOOP
FIL.
U401
AUDIO
(From filter)
3DB
ATTEN
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