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3. TECHNICAL BRIEF
Figure 3-2 Block Diagram of Receiver part of SI4205
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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