LG B2050, Service Manual

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3. TECHNICAL BRIEF
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Figure 3-2 SI4205 RECEIVER PART
B. Intermediate frequency (IF) and Demodulation
A quadrature image-reject mixer downconvertsthe 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 out-
put is amplified with an analog programmable gain amplifier (PGA), which is controlled with the
AGAIN[2:0] bits in register 05h (Figure3-2). The quadratureIF signal is digitized with high resolu-
tion 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 interfaceto standard analog ADC input baseband
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 dis-
tortion of blockers, and device 1/f noise.