LG U300, Service Manual

Page: 31 / 184

3.5.6 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 R118 plus R121, R111, and R120; the high band GSM π-pad is formed by R117 plus
R121, R109, 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. Since the RTR6250 TX_VCO_FB pin presents fairly
high impedance, an external terminating resistor is required (R121, 51Ω). A series capacitor (82 pF)
AC couples the feedback signal into the RTR6250 IC.
3.5.7 UMTS Rx RF filter (FL105)
An RF filter is located between the UMTS LNA and mixer. Insertion loss is important, but not as critical
as losses before the LNA. The most important parameters of this component include:
• Out-of-band rejection or attenuation levels, usually specified to meet these conditions:
- Far out-of-band signals - ranging from DC up to the first band of particular concern and from the
last band of particular concern to beyond three times the highest passband frequency.
- Tx-band leakage - the transmitter channel power, although attenuated by the duplexer, still
presents a cross-modulation threat in combination with Rx-band jammers. The RF filter must
provide rejection of this Tx-band leakage.
- Other frequencies of particular concern . bands known to include other wireless transmitters that
may deliver significant power levels to the receiver input.
• Phase and amplitude balance - the ZIF architecture requires well-balanced differential inputs to the
RFR6250 IC. This is accomplished by the RF filter which takes a single-ended output from the
RFL6250 IC and provides differential outputs having nominal 180° phase separation. Phase and/or
amplitude imbalance causes degraded common-mode rejection and second-order nonlinearity, so
their requirements are specified jointly.
-
±
3 degrees and
±
1 dB
- -12 to + 3 degrees and
±
0.7 dB
Of course, passband ripple and return loss are still important in all cases for the same reasons
3. TECHNICAL BRIEF
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