Transmitter - LG U8130 Service Manual

3. TECHNICAL BRIEF

3.8.2 Transmitter

Analogue differential signals (currents), representing I and Q, are sent to the radio ASCI Wivi (W-
CDMA Transmitter ASIC) from the D/A converter in Wanda (W-CDMA digital base-band coprocessor
ASIC). The signals are filtered in a reconstruction filter and then modulated up to 380MHz (using the
IFLO signal). The signal is then amplified in a VGA and filtered in an external filter (an LC filter). After
filtering, the signal is mixed to its final frequency (using the RFLO) and amplified in a differential output
RF buffer with two different gain settings (high gain or low gain).
The differential RF signal is fed into a SAW filter with a single ended output, and is then amplified in a
stand-alone RF buffer. After the RF buffer, the signal is filtered again in a SAW filter before it is fed to
the PA (Power Amplifier).
In the PA the signal is amplified for the last time before leaving the radio. After the PA, the signal is
sent through an isolator and through the duplexer, which directs the transmit signal to the antenna
connector via the antenna switch.
The PA has variable supply voltage, which adapts itself by means of a control loop so that the linearity
of the PA is kept constant. The variable supply voltage is provided from the battery through a DC/DC
converter and a signal linearity detector sits at the PA output. The detected signal at the PA output is
compared with a reference (supplied by the Vincenne, the mixed-signal circuit ASIC), and the error
signal is used in a loop filter, which provides the control signal to the DC/DC converter.
A. Reconstruction Filters
The reconstruction filters consist of input buffers that provide the correct DC biasing for the preceding
DAC in the digital baseband controller, and a low-pass filter for removing the unwanted high frequency
components from the baseband input waveform.
The filter inputs are adapted for use with a current-source type of input signal.
B. IQ-modulator
The IQ-modulator receives the incoming I and Q analog baseband signals at baseband frequency and
converts them to an intermediate frequency of 380MHz.
C. Variable Gain Amplifier (VGA)
Comprising two cascaded variable gain amplifiers, the VGA-together with the RF mixer- controls the
power of the transmitter.
The first of these two amplifiers, the so-called QVGA, enables fine-tuning of the transmitter by varying
the gain in 0.25dB steps, that is 0/0.25/0.5/0.75dB. The second amplifier provides a 54dB gain range
in 1 dB steps (54steps = 55 levels).
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