Receiver Section - Samsung SCH-470 Service Manual

Circuit Description

5-2 Receiver Section

LOW NOISE AMPLIFIER (LNA, Q302)
The low noise amplifier amplifies a weak signal
received from the base station to obtain the
optimum scvel (Noise figure = 1.5 dB,
Gain = 16 dB).
RADIO FREQUENCY BAND PASS FILTER
(RF BPF)
The RF BPF accepts only a specific frequency (881
°æ 12.5 MHz) from the signal received from the
base station. The band width is 25 MHz.
DOWN CONVERTER (MIXER, U302)
First local signal is applied to this down converter.
The down converter transfers the signal amplified
at the LNA into 85.38 MHz IF signal. 85.38 MHz IF
signal is made by subtracting 881 °æ12.5 MHz RF
signal from 966 °æ12.5 MHz first local signal.
AUTOMATIC GAIN CONTROLLER (AGC)
AMP U303)
85.38 MHz IF signal is applied to IF AGC amp, the
IF AGC output level is applied to BBA (Baseband
Analog ASIC). The IF AGC amp (U302) keeps the
signal at a constant level by controlling the gain.
Dynamic range is 90dB, up gain +45dB, and down
gain -45dB.
5-4
IF BAND PASS FILTER (FOR CDMA)
IF SAW BPF (F303) is used for CDMA system
having 1.23 MHz wideband and °æ630 kHz
bandwidth. The filter also attenuates the image
product generated at the mixer.
BUFFER AMP (Q385)
Buffer (Q385) amplifiers signal to be applied to the
local input of the down converter (U301) when a
phase is locked between VCO (U341) and PLL IC
(U342).
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