7-18 Low Band, 800 MHz, PassPort & 900 MHz Theory of Operation: 900 MHz Receiver
U103 and its associated components are part of the temperature cut back circuitry. It senses the
printed circuit board temperature around the transmitter circuits and output a DC voltage to the
PCIC. If the DC voltage produced exceeds the set threshold in the PCIC, the transmitter output
power will be reduced so as to reduce the transmitter temperature.
7.11 900 MHz Receiver
Figure 7-11. 900 MHz Receiver Block Diagram
7.11.1 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
109.65 MHz IF signal.
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
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