LG U8210, Руководство по обслуживанию, Страница: 30 / 231

3. TECHNICAL BRIEF
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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 Antenna switch module (U207 : LMSP43MA-288)
The antenna switch module allows multiple operating bands and modes to share the same antenna. In
the U8210 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.
3.5.2 UMTS duplexer (U202 : DMF1950IHC)
A UMTS duplexer splits a single operating band into receive and transmit paths. Important
performance requirements include:
-. Insertion loss . this component is also in the receive and transmit paths; In the U8210 typical losses:
UMTS Tx = 1.2 dB, UMTS Rx = 1.5 dB.
-. Out-of-band rejection or attenuation . the duplexer provides input selectivity for the receiver, output
filtering for the transmitter, and isolation between the two. Rejection levels for both paths are
specified over a number of frequency ranges. Two Tx-to-Rx isolation levels are critical to receiver
performance:
-. Rx-band isolation . the transmitter is specified for out-of-band noise falling into the Rx band. This
noise leaks from the transmit path into the receive path, and must be limited to avoid degrading
receiver sensitivity. The required Rx-band isolation depends on the PA out of-band noise levels and
Rx-band losses between the PA and LNA. Typical duplexer Rx band isolation value is 45 dB.
-. Tx-band isolation . the transmit channel power also leaks into the receiver. In this case, the leakage
is outside the receiver passband. but at a relatively high level It combines with Rx band jammers to
create cross-modulation products that fall inband to desensitize the receiver. The required Tx-band
isolation depends on the PA channel power and Tx-band losses between the PA and LNA. Typical
duplexer Tx-band isolation value is 50 dB.
-. Passband ripple . the loss of this fairly narrowband device is not flat across its passband. Passband
ripple increases the receive or transmit insertion loss at specific frequencies, creating performance
variations across the band’s channels, and should be controlled.