LG U300, Service Manual

Page: 23 / 184

3.3 WCDMA Mode
3.3.1 Receiver
The UMTS duplexer receiver output is routed to LNA circuits within the RFR6250 IC.
The LNA gain is dynamically controlled by the MSM6250A IC to cover full receiver dynamic range and
to save current consumption.
The UMTS LNA output is routed to the down conversion mixer inputs, in the RFR6250 IC, through a
band selection filter that transforms a single-ended 50-Ω source to differential 100-Ω load impedance
that is matched to the RFR6250 IC. The RFR input uses a differential configuration to improve second-
order inter-modulation and common mode rejection performance. The RFR6250 IC input stages include
MSM-controlled gain adjustments that further extend receiver dynamic range.
The amplifier output drives the RF port of the quadrature RF-to-baseband downconverter.
The down-converted baseband outputs are routed to low-pass filters (one I and one Q) having pass-
band and stop-band characteristics suitable for DS-WCDMA processing. The filter outputs are buffered
and passed on to the MSM6250A IC for further processing. This baseband interface is shared with the
RTR6250 GSM receiver outputs.
The RFR6250 IC includes LO generation and distribution circuitry to reduce off-chip component
requirements. The GPS RX LO source is created using the PLL control elements of the RTR6250
PLL2, via a discrete loop filter components, in tandem with the VCO in the RFR6250. Using only this
PLL signal, the RFR6250 LO generation and distribution circuits create the necessary LO signals for
the UMTS quadrature downconverter.
By definition, the ZIF down-converter requires F
LO
equal to F
RF
, and the RTR6250/RFR6250 design
achieves this without allowing F
VCO
to equal F
RF
.
3. TECHNICAL BRIEF
- 24 -
Z3X-BOX.COM