Samsung SCH-3500 series Service Manual page 25

Circuit Description
6-3 Receiver
(For AMPS)
LOW NOISE AMPLIFIER (LNA)
The low noise amplifier featuring 1.6 dB Noise
Figure and 20 dB gain amplifier a weak signal
receiver 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 85.38 MHz IF signal. 85.38 MHz IF
signal is made by subtracting 881 ±12.5 MHz RF
signal from 966 ± 12.5 MHz first local signal. The
LNA is Q102 and down converter is U102.
LOCAL BUFFER AMPLIFIER
Buffer (Q204) amplifies signal to be applied to the
local input of the down converter (U412) when a
phase is locked between U202 and U204.
RF BAND PASS FILTER (BPF)
The RF BPF (F105) passes only a specific frequency
(881.49±12.5MHz) from the signal received from the
mobile station. The band width is 25 MHz.
IF SAW BAND PASS FILTER
IF SAW BPF (F107) is used for AMPS system having
30kHz channel spacing and ±15 kHz bandwidth.
The filter also climinates the spurious signals at the
mixer.
SAMSUNG Proprietary-Contents may change without notice
VOLTAGE CONTROLLED OSCILLATOR
The VCO (U202) generates the signal having
966MHz center frequency and ±12.5 MHz deviation
with voltage control. PLL IC (U204) controls this
signal.
PHASE LOCKED LOOP (PLL)
Input reference frequency is generated at VCTCXO
(U205) and the divider signal is generated at VCO.
PLL compares the two signals and generates the
desired signal with a preprogrammed counter
which controls voltage.
VOLTAGE CONTROLLED TEMPERATURE
COMPENSATED CRYSTAL ASCILLATOR
It provides 19.68 MHz reference frequency to PLL
(U204) and IFR (U103), IFT(U501). A correct
frequency tuning is made by the voltage control.
DUPLEXER
Duplexer (F103) controls to transmit through the
antenna only the signals within acceptable Tx
frequency range (836 ±12.5 MHz). It also matches
LNA (Q102) input in receiving part and PA out in
transmitter part with the antenna.
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