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LGE Internal Use Only
Copyright © 2009 LG Electronics. Inc. All right reserved.
Only for training and service purposes
4. RF Circuit Technical Brief
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Baseband Processing (analog)
The amplified RF signal is converted by a quadrature demodulator to I and Q signals at baseband
frequency. The resulting inphase and quadrature signals are fed into the analog baseband low pass
filter (Chebyshev type). An optional, additional filter stage can be activated in order to provide the
required selectivity for narrowband blockers. The filter corner frequency is controlled by an on-chip
filter alignment circuit. This filter also provides the overall baseband PGC functionality. The differential
offset voltage after the demodulator is reduced by an on-chip DC compensation loop. The subsequent
last PGC amplifier stage provides 1 dB gain steps. All gains are selected by 3-wire bus program-ming.
4.3.2 Transmitter
The direct-up conversion transmitter for each band consists of:
•fully differential signal path
•configurable input stages (programmable AC and DC input voltages)
•analog channel filter including auto calibration circuit (w/o external components)
•automatic carrier adjustment also featuring compensation of residual I/Q DC offsets
•I/Q modulator including LO buffer and I/Q divider
•LO including on-chip VCO and synthesizer
•RF voltage-controlled gain amplifier (VGA)
For each band, the modulator performs a direct quadrature modulation of the baseband input signals
at I and Q. The internally divided RF VCO signal is split into two orthogonal carriers. The gain of the
VGA is controlled by the voltage at the control pin TXGC. The robustness against PCB and baseband
processor spurious as well as noise is improved by an integrated baseband filter (Butterworth type).
The RF output signal is available at the differential outputs TXOUTH,/TXOUTHX, TXOUTM/TXOUTMX,
and TXOUTL/TXOUTLX. The modulator supply voltage must also be applied to these open drain
differential outputs.
TX Output Power
The voltage at the VGA control pin TXGC (VGC) should be limited to ensure that the specified
maximum output power (minimum Poutmax) is not exceeded. The VGC value at which the specified
minimum Poutmax is reached varies from device to device. ACLR and other specifications are only
valid for output power levels below the specified minimum Poutmax.
TX Front End
An external power amplifier as well as duplex and interstage filters are needed to form a complete
transmit chain for each band.
Содержание KM555
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