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Copyright © 2009 LG Electronics. Inc. All right reserved.
Only for training and service purposes
LGE Internal Use Only
3. Technical Brief
BL20
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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 180and GSM 190bands 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 signals to the two downconverters.
The buffered 19.2 MHz TCXO or XO signal provides the synthesizerinput (REF), the frequency refrence to
which the PLL is phase and frequency locked. The refernce 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-lop wilforce 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.
