LG KU730 Service Manual page 32

3.5.7 GSM transmit VCO (U102 : MQW5V0C869M)
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.
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 R121 plus R124, R116 and R270; the high band GSM π-pad is formed by R120 plus
R124, R115 and R122. 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 (R124, 51 Ω). A series capacitor (82 pF)
AC couples the feedback signal into the RTR6250 IC.
3.5.8 UMTS Rx RF filter (FL101- B7827)
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
RFR6250E IC. This is accomplished by the RF filter which takes a single-ended output from the
RFR6250E 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
explained in the antenna switch module and duplexer sections.
3. TECHNICAL BRIEF
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