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

RECEIVER
3-POLE CERAMIC
RF
AMP
PRE-SELECTOR
T/R
SWITCH
HARMONIC
FILTER
DIRECTIONAL
COUPLER
Receiver Detailed
Functional Description
7-2 Theory of Operation
MIXER
3-POLE CERAMIC
PRE-SELECTOR
LO SIGNAL
PA
PA
SYNTHESIZER
LINEUP
LINEUP
Figure 7-1 800-MHz Radio Functional Block Diagram
The receiver is composed of four sections: receiver front end, receiver IF, receiver
back end and the audio signal filter IC (ASFIC) that is part of the controller
section.
The radio signal from the antenna switch in the PA section enters the first bandpass
filter (FL5203). The first bandpass filter has three poles, a 860-MHz center
frequency, a 20-MHz wide passband and a 40-dB rejection for image frequencies
(761 to 780 MHz).
After the first bandpass filter, the signal passes to a pair of hot-carrier limiting
diodes (D5303) placed in front of RF preamplifier Q5301. The hot-carrier diodes
limit strong signals to prevent them from overdriving the RF preamplifier and
damaging it.
The front end amplifier is a low noise amplifier that consists of Q5301. Its main
purpose is to set the noise figure of the receiver. The front end amplifier as well as
the IF amplifiers are shut off during transmit by K9.1 line via switch Q5230, to
provide isolation in talk-around mode.
BACK-END
2-POLE IF
2.1 MHZ
PENDULUM
16.8 MHZ
SYNTHESIZER
RX_TX
VCO
LOOP
FILTER
TA
VCO
AUDIO & LOGIC
AUDIO
PA
ASFIC
MOD
CPU CLOCK
SPI
CPU
MEMORY
KEYBOARD
DISPLAY
FRAC N
IC
MOD
POWER
CONTROL