Low If; Processing; Synthesizer Part - LG MG810c Service Manual

3. H/W Circuit Description

3.2.2. Low IF

A quadrate image-rejection mixer downconverts the RF signal to a 100kHz intermediate frequency (IF)
with the RFLO . The RFLO frequency is between 1849.8 and 1918.8 MHz, and is divided by two for
EGSM modes. The RFLO frequency is between 1804.9 and 1879.9 MHz, and is divided by one for
DCS modes. The RFLO frequency is between 1929.9 and 1989.9 MHz, and is divided by one for PCS
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. The quadrate IF signal is digitized with high
resolution A/D converters (ADCs). The Si4210[U401] down-converts the ADC output to baseband with
a digital 100kHz quadrate LO signal. Digital decimation and IIR filters perform channel selection to
remove blocking and reference interference signals. The response of the IIR filter is programmable to
a high selectivity setting(CSEL=0) or a low selectivity setting (CSEL=1). After channel selection, the
digital output is scaled with digital PGA, which is controlled with the DGAIN[5:0] bits in register 05h.
3.2.3. Demodulator and Baseband Processing
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.

3.3 Synthesizer Part

The Si4210[U401] integrates two complete PLLs including VCOs, varactors, resonators, loop filters,
reference and VCO dividers, and phase detectors. The RF PLL uses two multiplexed VCOs. The RF1
VCO is used for receive mode, and the RF2 VCO is used for transmit mode. The IF PLL is used only
during transmit mode. All VCO tuning inductors are also integrated. The IF and RF output frequencies
are set by programming the N-Divider registers, NRF1, NRF2, and NIF. Programming the N-Divider
register for either RF1 or RF2 automatically selects the proper VCO. The output frequency of each
PLL is as follows:
F = N x fφ
OUT
A programmable divider in the input stage allows either a 13 or 26 MHz reference requency depending
on the choice of crystal. A 26 MHz reference clock can be divided by 2 using the DIV2 bit in Register
31h. The RF PLL phase detector update rate (fφ) can be programmed with the RFUP bit in register
31h to either fφ= 100 kHz or fφ = 200 kHz. The IF PLL always uses fφ = 200 kHz. Receive mode
should use fφ = 100 kHz in DCS 1800 and PCS 1900 bands, and fφ = 200 kHz in the E-GSM 900
bands. Transmit modes should always use fφ = 200 kHz.
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