7-2
Theory of Operation
Figure 7-1 800-MHz Radio Functional Block Diagram
Receiver Detailed
Functional Description
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.
CPU CLOCK
MOD
SPI
MOD
2.1 MHZ
SYNTHESIZER
DIRECTIONAL
COUPLER
PRE-SELECTOR
3-POLE CERAMIC
2-POLE IF
BACK-END
ASFIC
CPU
KEYBOARD
DISPLAY
MEMORY
POWER
CONTROL
HARMONIC
FILTER
AUDIO
PA
RF
AMP
PA
LINEUP
FRAC N
IC
LOOP
FILTER
T/R
SWITCH
PENDULUM
16.8 MHZ
RX_TX
VCO
TA
VCO
PRE-SELECTOR
3-POLE CERAMIC
SYNTHESIZER
LINEUP
PA
RECEIVER
AUDIO & LOGIC
MIXER
LO SIGNAL
