Off-Chip Rf Components; Front End Module(U1006) - LG U8500 Service Manual

3. TECHNICAL BRIEF
The off-chip loop filter allows optimization of key PLL performance characteristics (stability, transitory
response, settling time, and phase noise) for different applications. Guidelines are provided in the next
subsection for proper implementation of this critical circuit.

3.5 Off-chip RF Components

3.5.1 Front End Module(U1006)

Front End module integrates antenna switch module and GSM Rx filter.
The antenna switch module allows multiple operating bands and modes to share the same antenna. In
the U8500 design, a common antenna connects to one of six paths: 1) UMTS-2100 Rx/Tx, 2) EGSM
Rx, 3) DCS-1800 Rx, 4) PCS-1900 Rx, 5)EGSM Tx, and 6) DCS-1800, PCS-1900 Tx. UMTS
operation requires simultaneous reception and transmission, so the UMTS Rx/Tx connection is routed
to a duplexer that separates receive and transmit signals. GSM band of operation is time division
duplexed, so only the receiver or transmitter is active at any time and a frequency duplexer is not
required. The module includes lowpass filters for the GSM bands transmit paths to reduce out-of-band
emissions, PA harmonics in particular.
The GSM mode RF filters are located before their LNAs, so their insertion losses are extremely critical
(1.5 dB typical). Other important parameters are:
• Out-of-band rejection or attenuation levels
- 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.
- Frequencies of particular concern . bands known to include other wireless transmitters that may
deliver significant power levels to the receiver input.
- GSM band receivers operate while the handset transmitters are off so there are no Tx-band
leakage attenuation requirements.
• Phase and amplitude balance - the UMTS discussion presented above applies for GSM bands as
well. See the data sheet for specific values. 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.
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