LG KB770 Service Manual page 26

3.5.2 UMTS Dupexer(FL102 : ACMD-7602)
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 KB770 typical losses : UMTS2100_ Tx = 1.6 dB, UMTS2100_ Rx = 2.0 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. Minimum duplexer Rx band isolation value is about 46.7 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 in-band to desensitize the receiver. The required Tx-band isolation
depends on the PA channel power and Tx-band losses between the PA and LNA. Minimum duplexer Tx-
band isolation value is about 51.7dB.
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.
Return loss, minimize mismatch losses with typical return losses of 10 dB or more (VSWR <2:1).
Power handling, high power levels in the transmit path must be accommodated without degraded
performance. The specified level depends on the operating band class and mobile station class (per the
applicable standard), as well as circuit losses and antenna EIRP. Several duplexer characteristics depend
upon its source and load impedances. QUALCOMM strongly recommends an isolator be used between
the UMTS PA and duplexer to assure proper performance.
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3. TECHNICAL BRIEF
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