LG U250, Service Manual

Page: 21 / 175

3.2.2 GSM Transmitter
The RTR6275 transmitter outputs(DA_HB2_OUT and DA_LB1_OUT)include on-chip output matching
inductors. The 50ohm output impedance is achieved by adding a series capacitor at the output pins.
The capacitor value may be optimized for specific applications and PCB characteristics based on pass-
band symmetry about the band center frequency, the suggested starting value is shown in Figure1.3.
The RTR6275 IC is able to support GSM 900 and GSM 1800/1900 mode transmitting. This design
guideline shows a tri-band GSM application.
Both high-band and low band outputs are followed by resistive pads to ensure that the load presented
to the outputs remains close to 50ohm. The low-band GSM Tx path also includes a Tx-band SAW filter
to remove noise-spurious components and noise that would be amplified by the PA and appear in the
GSM Rx band
3. TECHNICAL BRIEF
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91 91
6pF
100 100
51
12pF 39
Ω
Ω
Ω
Ω
Ω
Ω
[Fig 1.3] GSM Transmitter matching