LG VX8560 Service Manual page 38

VX8560
1.2.4 Down-converter Mixers (U200)
The QSC6075 device performs signal down-conversion for Cellular, PCS and GPS tri-band applications. It contains
all the circuitry (with the exception of external filters) needed to support conversion of received RF signals to
baseband signals. The three downconverting Mixers (Cellular, PCS and GPS), and an LO Buffer Amplifier to buffer
the RF VCO to the RF Transmit Upconverter. The GPS LNA & mixers offer the most advanced and integrated
CDMA Rx solution designed to meet cascaded Noise Figure (NF) and Third-order Intercept Point (IIP3)
requirements of IS-98C and J-STD-018 specifications for Sensitivity, Two-Tone Intermodulation, and Single-tone
Desense.
Operation modes and band selection are specially controlled from the Qualcomm Single Chip QSC6075.
1.2.5 Rx RF Dual SAW FILTERs(F105)
The main function of Rx RF SAW filter is to attenuate mobile phone spurious frequency, attenuate direct RF
frequency pick up, attenuate noise at the image frequency originating in or amplified by the LNA and suppress
second harmonic originating in the LNA. The Rx RF SAW filter usually called image filter.
1.2.6 RF Receiver(U200)
The circuit functions of the RF Receiver(in QSC6075) include Rx Automatic Gain Controller (AGC) with 90 dB
dynamic range, quadrature RF mixers, down-conversion mixer from RF to base-band, low pass filters and Analog to
Digital Converters (ADC) for converting to digital base-band. The RFR includes clock generators that drive the
digital processor and a VCO which generates the LO frequency for base-band down-conversion.
Switching system is located in front of the RFR RX_IN_C_LB and RX_IN_C_HB terminal and is for band selection
between cellular and PCS. The Rx AGC either amplifies or attenuates the received CDMA RF signal to provide a
constant-amplitude signal to the I/Q down-converter. The RF output of the Rx AGC amplifier separate into I-channel
and Q-channel base-band components and down-converted by mixer with quadrature LO. LO signals are generated
by a Voltage Controlled Oscillator (VCO) and frequency stabilized by external varactor-tuned resonant tank circuit.
The I/Q down converter outputs the CDMA signals at baseband frequency. Low-pass filtering enables the receiver to
select the desired baseband signals from the effects of unwanted noise or adjacent-channel interference. I/Q base
band components are converted to digital signals by two identical 4-bit ADCs.
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