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Circuit Description
6-2 Receiver Section
LOW NOISE AMPLIFIER (LNA)
The low noise amplifier featuring 1.5dB and 16 dB gain amplifies a weak signal received from the base station to obtain the optimum signal level.
IF SAW BAND PASS FILTER FOR CDMA
IF SAW BPF (F303) is used for CDMA system having 1.23 MHz wide band and 630 kHz band width. The filter also eliminates the image product generated at the mixer.
DOWN CONVERTER (MIXER) BUFFER
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 MHz IF signal is made by subtracting 881 12.5 MHz RF signal from 966 12.5MHz first local signal. Buffer (Q342) amplifies 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).
RX IF AUTOMATIC GAIN CONTROLLER (AGC) AMP
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) keep the signal at a constant level by controlling the gain. Dynamic range is 90 dB, up gain +45dB, and down gain -45dB.
VOLTAGE CONTROLLED OSCILLATOR
The VCO (U341) generates the signal having 966 MHz center frequency and 12.5 MHz deviation with the voltage control. PLL IC (U342) controls this signal.
PHASE LOCKED LOOP (PLL) RF BAND PASS FILTER (BPF)
The RF BPF (F302) accepts only a specific frequency (881 12.5MHz) from the signal received from the mobile station. The band width is 25 MHz. Input reference frequency is generated at VC_TCXO (U343) and the divided signal is generated at VCO (U341). PLL compares the two signals and generates the desired signal with a pre-programmed counter which controls voltage.
IF SAW BAND PASS FILTER FOR AMPS
IF SAW BPF (F304) is used for AMPS system having 30 kHz channel spacing and 15 kHz band width. The filter also eliminates the image product generated at the mixer.
VOLTAGE CONTROLLED TEMPERATURE COMPENSATED CRYSTAL OSCILLATOR
It provides 19.68 MHz reference frequency to the mobile main set. A correct frequency tuning is made by the voltage control.
THERMISTOR
The thermistor (U371) detects temperature. It is used to compensate active component characteristics due to the temperature difference.
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