Umts Mode - LG CU500 Service Manual

3. TECHNICAL BRIEF
The waveform at the dual Tx VCO output is the GMSK or 8-PSK-modulated signal centered at the
desired GSM channel frequency. A phase-locked loop circuit is used to translate the GMSK or 8-PSK-
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 inband
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
(GSM850/GSM900 or DCS/PCS) is selected by the MSM device GPIO control (GSM_PA_BAND).

3.3 UMTS Mode

3.3.1 Receiver
The UMTS duplexer receiver output is routed to LNA circuits within the RFL6202 device. These LNA
functions are removed from the RFR6202 IC to improve mixer LO to RF isolation - a critical parameter
in the Zero-IF architecture. Isolation is further improved using high reverse isolation circuits in the LNA
designs. The LNA gain is controlled by the MSM device to minimize DC power consumption while
achieving the desired RF performance.
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