Receiver Front End; Receiver Front-End; 800 Mhz Receiver - Motorola GP Series Service Manual

800 MHz Receiver

3.0 800 MHz Receiver
The receiver functions are shown in Figure 2-20 and are described in the paragraphs that follow.
Antenna
RFJack
3.1

Receiver Front-End

( Refer to Figure 2-2 and the UHF Receiver Front End schematic diagram)
The RF signal is received by the antenna and applied to a low-pass filter. For 800MHz, the filter
consists of L101, L102, C104, C105, C106, C107, C109. The filtered RF signal is passed through the
antenna switch. The antenna switch circuit consists of two PIN diodes(CR101 and CR102) and a pi
network (C109, L103 and C110).The signal is then applied to a fixed tuned ceramic bandpass filter,
FL300.
The output of the bandpass filter is coupled to the RF amplifier transistor Q302 via C300. The RF
amplifier provides a gain of approximately 12 dB. After being amplified by the RF amplifier, the RF
signal is further filtered by a second fixed tuned ceramic bandpass filter, FL301.
Both the pre and post-RF amplifier ceramic filters have similar responses. The insertion loss of each
filter across the 851-870MHz band is typically 1.8dB.
The output of the post-RF amplifier filter is connected to the passive double balanced mixer, U301.
After mixing with the first LO signal from the voltage controlled oscillator (VCO) using low side
injection, the RF signal is down-converted to the 109.65MHz IF signal.
Pin Diode 3-Pole
Antenna
Ceramic
Switch
Block Filter
Recovered Audio
Reference Clock
Figure 2-2 800 MHz Receiver Block Diagram.
3-Pole
RF Ceramic
Amp Block Filter
First LO
from FGU
Squelch
RSSI
16.8 MHz
Crystal
Mixer
Amp
Filter
AGC
Processing
Demodulator
U351
Synthesizer
IF IC
SPI Bus
2-3
IF
Second
LO VCO