LG L601i Service Manual page 31

3. TECHNICAL BRIEF
Since the RTR6250 TX_VCO_FB pin presents fairly high impedance, an external terminating resistor
is required (R123, 51 Ω). A series capacitor (82 pF) AC couples the feedback signal into the RTR6250
IC.
3.5.8 UMTS Rx RF filter (FL101)
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
3.5.9 VCTCXO (X100)
The Voltage Controlled Temperature Compensated Crystal Oscillator (VCTCXO) provides the
reference frequency for all RFIC synthesizers as well as clock generation functions within the
MSM6250A IC. The 6250-series chipset requires a 19.2 MHz nominal VCTCXO frequency. The
oscillator frequency is controlled by the MSM6250A's TRK_LO_ADJ pulse density modulated signal in
the same manner as the transmit gain control.
The filtered PDM signal results in an analog control signal into the VCTCXO tuning port whose voltage
is directly proportional to the density of the digital bit stream. The MSM device varies the pulse density
to change the analog control voltage that sets the oscillator frequency - all within a feedback control
loop that minimizes handset frequency drift relative to the network.
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