Samsung SCH-2000, Service Manual, Page: 14 / 97

Circuit Description
4-4 Samsung Electronics
4-2 Receiver Section
LOW NOISE AMPLIFIER (LNA)
The low noise amplifier featuring 1.6dB Noise
Figure and 19dB gain amplifies a weak signal
received from the base station to obtain the
optimum signal level.
DOWN CONVERTER (MIXER)
First local signal is applied to this down converter.
The down converter transfers the signal amplified
at the LNA into 210.38 MHz IF signal. 210.38MHz
IF signal is made by subtracting 1960
°æ
30MHz RF
signal from 1750
°æ
30MHz first local signal. The
LNA is Q303 and down converter is U302.
RF IF AUTOMATIC GAIN CONTROLLER
(AGC) AMP
210.38 MHz IF signal is applied to IF AGC
amp,the AGC output level is applied to BBA (Base
Band Asic). The AGC amp (U303) keeps the signal
at a constant level by controlling the gain.
Dynamic range is 90dB, up gain +45dB, and down
gain -45dB.
RF BAND PASS FILTER (BPF)
The RF BPF(F302) accepts only a specific
frequency (1960
°æ
30MHz) from the signal
received from the mobile station. The band width
is 60MHz.
IF SAW BAND PASS FILTER FOR CDMA
IF SAW BPF (F303) is used for CDMA system
having 1.23 MHz wide band and
°æ
630kHz band
width. The filter also eliminates the image product
generated at the mixer.
BUFFER
Buffer (Q311) amplifies signal to be applied to the
local input of the down converter (U302) when a
phase is locked between U304 and U305.
VOLTAGE CONTROLLED OSCILLATOR
The VCO (U305) generates the signal having
1750MHz center frequency and
°æ
30MHz
deviation with the voltage control. PLL IC(U304)
controls this signal.
PHASE LOCKED LOOP (PLL)
Input reference frequency is generated at
VCTCXO (U306) and the signal is generated at
VCO. PLL compares the two signals and generate
the desired signal with a pre-programmed counter
which controls voltage.
VOLTAGE CONTROLLED TEMPERATURE
COMPENSATED CRYSTAL OSCILLATOR
It provides 19.68MHz reference frequency to PLL
(U304) and BBA (U401). A correct frequency
tuning is made by the voltage control.
DUPLEXER
Duplexer (F301) controls to transmit through the
antenna only the signals within acceptable Tx
frequency range (1880
°æ
30MHz) and to receive
through the antenna only the signals within
acceptable Rx frequency range (1960
°æ
30MHz). It
also matches LNA(Q303) input in receiving part
and PA output in transmitter part with the
antenna.