LG C3400 Service Manual page 14

3. TECHNICAL BRIEF
Figure. 3-2 SI4205 RECEIVER PART
B. Intermediate frequency (IF) and Demodulation
A quadrature image-reject mixer downconverts the RF signal to a 100KHz intermediate
frequency (IF) with the RFLO from the frequency synthesizer. The RFLO frequency is
between 1737.8 to 1989.9 MHz, and is internally divided by 2 for GSM 850 and E-GSM 900
modes. The mixer output is amplified with an analog programmable gain amplifier (PGA),
which is controlled with the AGAIN[2:0] bits in register 05h (Figure3-2). The quadrature IF
signal is digitized with high resolution A/D converters (ADCs).
The ADC output is downconverted to baseband with a digital 100KHz quadrature LO signal.
Digital decimation and IIR filters perform channel selection to remove blocking and reference
interference signals. The selectivity setting (CSEL=0) or a low selectivity setting (CSEL=1).
The low selectivity filter has a flatter group channelization filter is in the baseband chip. After
channel selection, the digital output is scaled with a digital PGA, which is controlled with the
DGAIN [5:0] bits in register 05h.
The amplified digital output signal go through with DACs that drive a differential analog signal
onto the RXIP,RXIN,RXQP and RXQN pins to interface to standard analog ADC input
baseband ICs. No special processing is required in the baseband for offset compensation or
extended dynamic range.
Compared to a direct-conversion architecture, the low-IF architecture has a much greater
degree of immunity to dc offsets that can arise from RF local oscillator(RFLO) self-mixing,
2nd order distortion of blockers, and device 1/f noise.
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