B. Intermediatefrequency (IF) and Demodulation
A quadratureimage-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 downconvertedto basebandwith a digital 100KHz quadratureLO signal. Digital
decimation and IIR filters perform channel selection to remove blocking and reference interfer-
ence signals. The selectivity setting (CSEL=0) or a low selectivity setting (CSEL=1). The low
selectivity filter hasa flatter group channelizationfilter is in the basebandchip. After channel selec-
tion, 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 DACsthat drive a differential analog signal
onto the RXIP,RXIN,RXQP and RXQN pins to interfaceto standard analog ADC input
basebandICs. No special processing is required in the basebandfor 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.
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