LG GD910 Service Manual page 53

4. RF Circuit Technical Brief
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.
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GD910 service manual
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