LG L601i Service Manual page 30

3. TECHNICAL BRIEF
RF input and output ports of the SKY77328 are internally matched to a 50 Ω load to reduce the number of
external components for a quad-band design. Extremely low leakage current (2.5 µA, typical) of the dual PA
module maximizes handset standby time. The SKY77328 also contains band-select switching circuitry to
select GSM (logic 0) or DCS/PCS (logic 1) as determined from the Band Select (BS) signal. In Figure 3.5.6-
1 below, the BS pin selects the PA output (DCS/PCS OUT or GSM850/900 OUT) and the Analog Power
Control (VAPC) controls the level of output power. The VBATT pin connects to an internal current-sense
resistor and interfaces to an integrated power amplifier control (iPAC™) function, which is insensitive to
variations in temperature, power supply, process, and input power. The ENABLE input allows initial turn-on
of PAM circuitry to minimize battery drain.
Figure 3.5.6-1 GSM PA functional block diagram
3.5.7 GSM transmit VCO (U103)
The dual Tx VCO is a key component within the GSM OPLL. This VCO performance directly impacts
PLL and transmitter performance. VCO specifications refer to muRata MQW5V0C869M datasheet.
The dual Tx VCO outputs, one for Low-band GSM and one for high band, drive a resistive network that
splits the active signal into two signals: 1) the input to the active PA . this is the low loss path, and 2)
the OPLL feedback signal . this is the high loss path. See Figure 8-1 for recommended topology and
resistor values.
The losses from the VCO outputs to the PA inputs must be factored into the output chain.s power
budget. Each path includes a π-pad that introduces approximately a 3-dB loss. The low band GSM π-
pad is formed by R120 plus R123, R115, and R122; the high band GSM π-pad is formed by R121 plus
R123, R114, and R119. One leg of each π-pad is used to couple the VCO output to form the feedback
path as described below.
For a given VCO output drive level, the loss to the RTR6250 input must assure the specified input
level is achieved (-18 to -12 dBm). Large resistors included in the π-pads are used to lightly couple off
the VCO outputs to create the feedback signal.
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