Service manual

2-36
2.22 900 MHz Receiver
Figure 2-25: 900 MHz Receiver Block Diagram
2.22.1 Receiver Front-End
The RF signal is received by the antenna and applied to a low-pass filter. For 900 MHz, the filter
consists of L104, L105, C114, C115, C124, C125, and C126. 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 (C115, L109, and C138). 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 14 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 935-941 MHz band is less than 2 dB.
The output of the post-RF amplifier filter is connected to the passive double balanced mixer, U301,
through matching components C321, and L311. 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.
The IF signal coming out of the mixer is transferred to the crystal filter (FL350) through a resistor pad
and a diplexer (C312, and L306). Matching to the input of the crystal filter is provided by L353,L354,
C377, and C378. The crystal filter provides some of the necessary selectivity, and intermodulation
protection.
Demodulator
Synthesizer
Crystal
Filter
Mixer
RF
Amp
IF
Amp
3-Pole
Ceramic
Block Filter
3-Pole
Ceramic
Block Filter
Antenna
First LO
from FGU
Recovered Audio
Squelch
RSSI
IF
IC
SPI Bus
16.8 MHz
Reference Clock
Second
LO VCO
RFJack
Pin Diode
Antenna
Switch
AGC
Processing
U351