LG TracFone LG840G Service Manual page 27

3. TECHNICAL BRIEF
Dedicated GSM configuration (two GSM LNAs)
In this configuration, the GSM 850 and GSM 1900 bands do not pass through the UMTS duplexers.
Instead, the two GSM LNA inputs are shared: the GSM 850 and GSM 900 bands share the low-band GSM LNA, and the
GSM 1800 and GSM 1900 bands share the high-band LNA.
Four switch module outputs are required, each driving its own GSM Rx path. A two-way SAW filter takes the two low-
band (or high-band) single-ended inputs from the antenna switch and provides one filtered, differential output that
drives the appropriate QSC LNA input.
Beyond the LNA inputs, this GSM receiver configuration is identical to the paths described earlier for the shared UMTS/
GSM configuration.
3.3.3 Rx LO circuits
The QSC62x0 device integrates all of the frequency synthesizer functions that generate the UMTS and GSM receive LO
signals (UHF local oscillator, PLL circuits, and loop filter), plus the distribution circuits that deliver the quadrature LO sig-
nals to the two downconverters.
The buffered 19.2 MHz TCXO or XO signal provides the synthesizer input (REF), the frequency reference to which the PLL
is phase and frequency locked. The reference is divided to create a fixed frequency input to the phase detector, FR.
The other phase detector input (FV) varies as the loop acquires a lock and is generated by dividing the local oscillator
output frequency using the feedback path's counter. The closed-loop will force FV to equal FR when locked.
If the loop is not locked, the error between FV and FR will create an error signal. This error signal is filtered by the loop
filter and applied to the local oscillator, tuning the output frequency so that the error is decreased. Ultimately the loop
forces the error to approach zero and the PLL is phase and frequency locked.
LGE Internal Use Only - 27 - Copyright © 2012 LG Electronics. Inc. All right reserved.
Only for training and service purposes