Motorola E685, Руководство по обслуживанию, Страница: 25 / 97

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3-3. Tx Part
1. RFT 6120 (IC101)
The RFT6120 cellular-only Baseband-to-RF transmit processor performs all transmit(Tx) signal
processing function required between digital baseband and the power amplifier(PA) for cdma2000,
IS-95A/B CDMA Cellular and TSB-74 single-band applications. The RFT6120 use Qualcomm’s
Zero-IF transmitter signal path, from analog baseband to RF driver amplifier, for single-band
CDMA Cellular (IS-95 and cdma2000).
The RFT6120 connects directly with Qualcomm’s MSM6100 utilizing an analog baseband interface.
The baseband quadrature signals are upconverted to the cellular frequency band and amplified to
provide signal drive capability to the PA.
Sophisticated Tx LO circuits provide frequency plan flexibility and are completely implemented
on-chip except for the loop filter (two resistors and two capacitors). The RFT6120 integrates the
Rx PLL as well (compatible with the RFR612x receiver IC), fulfilling all handset PLL requirements
without an additional PLL IC. Most transmit LO signals are generated using on-chip VCO circuits.
Virtually all the transmitter LO synthesizer is included within the RFT6120 IC ; only the loop filter is
off-chip, allowing customization for specific applications. The Phase-Locked Loop(PLL) circuits
include a reference divider, phase detector, charge pump, feedback divider, and digital logic that
generate LOCK status. The entire Tx VCO is on-chip as well. The RFT6120 IC integrates
significant Tx LO generation and distribution circuits on-chip. These circuits operate in various
modes to yield a highly flexible quadrature Tx LO output that drives the Cellular up converter. A
separate RX PLL circuit is integrated on-chip to support the receiver’s RF-to-baseband
downconverter.
The transmitter’s analog signal originates in the MSM’s I and Q Digital-to-Analog
Converters(DACs).
An external resistor sets several RFT6120 IC internal operating currents (charge pump currents,
signal path bias currents). Pin 6 (R_BIAS) connects this external resistor to the bias control
circuits. An 11.3 kΩ, 1% resistor must be connected between pin 6 and ground. The Tx gain is
controlled by a single analog control voltage presented at pin 21 (VCONTROL). This analog
voltage is developed by RC lowpass filtering a pulse density modulated signal from the MSM. A
total AGC range greater than 85 dB will be achieved with VCONTROL voltages ranging from
0.05V
DDM
to 0.95V
DDM
.
The RFT6120 functionality is specifically controlled from the MSM6100 via the three-line Auxiliary
Serial Bus Interface(Aux-SBI), allowing efficient initialization, control of device operating modes
and parameters, verification of programmed parameters, and status reports.