LG U300, Service Manual

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3. TECHNICAL BRIEF
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The waveform at the dual Tx VCO output is the GMSK-modulated signal centered at the desired GSM
channel frequency. A phase-locked loop circuit is used to translate the GMSK-modulated signal from IF
to RF primarily for two reasons:
1. Phase-locked loops provide a lowpass filter function from the reference input to the VCO output. This
results in a bandpass function centered at the desired channel frequency that provides steep, well-
controlled rejection of the out-of-band spectrum.
2. The resulting output bandpass function is virtually unchanged as the transmitter is tuned over
channels spanning the GSM operating band.
The PA is a key component in any transmitter chain and must complement the rest of the transmitter
precisely. For GSM band operation, the closed-loop transmit power control functions add even more
requirements relative to the UMTS PA. In addition to gain control and switching requirements, the usual
RF parameters such as gain, output power level, several output spectrum requirements, and power
supply current are critical.
The gain must be sufficient and variable to deliver the desired transmitter output power given the VCO
output level, the subsequent passive devices’ losses, and the control set point. The maximum and
minimum transmitter output power levels depend upon the operating band class and mobile station
class per the applicable standard. Transmitter timing requirements and in-band and out-of-band
emissions, all dominated by the PA, are also specified by the applicable standard.
The active dual Tx VCO output is applied to the dual power amplifier to continue the transmit path, and
feedback to the RTR6250 IC to complete the frequency control loop.
The PA operating band (EGSM or DCS/PCS) is selected by the MSM device GPIO control
(GSM_PA_BAND).
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