Motorola 38C V100, Service Manual, Page: 13 / 83

MOTOROLA CONFIDENTIAL - PROPRIETARY
6881036B20 November 20, 2000 9
Level 3 Service Manual Circuit Description
connector. The display is made up of glass with polarizers, a display driver and
transflector.
Transceiver
Receive
The RF signal from the base station is received through antenna A1 on the antenna
board to J3 on the transceiver board or from the RF phasing connector TP1 . The
signal is fed to Pin 10 or to Pin 3 respectively of RF switch U150 . The switch acts
as an isolation between TX and RX. RF switch control is provided by U151 which
determines whether the switch is opened for TX or RX, and if RF is passed to the
Aux RF port or the antenna. RF switching is managed by the following signals:
TX_EN and RX_EN are produced by Whitecap IC U800, Pin C1 and Pin E3
respectively. U151 is supported by the voltages FILTERED -5V (From -5V ( U903 ))
and RF_V1( Q201 ).
If a receive signal is present (using GSM 900, for example) in the RF switch, and
providing RX275_GSM_PCS (
Q2101 ) is high, the receive signal will be passed to
band pass filter FL400 which will pass the GSM 900 receive frequency.
RX275_GSM_PCS also selects the PCS 1900 frequency passed through FL2400 .
The DCS 1800 frequency is selected by RX275_DCS ( Q110 ) and passed through
FL1400
The PCS 1900 and DCS 1800 signals are fed to DCS/PCS select switch U400 .
RVCO_PCS and RVCO_DCS ( Q1100 ) then selects the appropriate signal, with
output tuning provided by L1411 and C1411 for DCS and C2411 for PCS.
The selected signal is fed to a low-noise amplifier circuit. Since this circuit is critical
for achieving a very low signal-to-noise ratio, a large amount of external frequency
matching and noise reduction circuitry is employed around the actual amplifiers
Q400 for GSM (supported by RX275_GSM ( Q110 )) and Q1400 for DCS / PCS
(supported by RX275_DPCS ( Q2102 )).
The signal is then fed to FL1401 (For GSM 1800 / 1900) or FL401 (For GSM 900)
where harmonics and other unwanted frequencies are removed.
The amplified signal is injected into the base of dual transistor mixer Q450 . Both
mixers are supported by RX275 ( Q112 ). The tuned-emitter biasing voltage is
provided by RX275_GSM ( Q110 ) and RX275_DPCS ( Q2102 ).
RX VCO U250 is driven by the Magic IC U200 Pin A1 CP_RX, which is programmed
by the Whitecap IC MQSPI bus. Power is supplied by RVCO_275 ( GPO4
through
Q1102 ).
The generated RX VCO signal is split, with part going back to the Magic IC U200
Pin A3 to serve as the feedback for the RX VCO phase-locked loop. The other part
Table 1. RF Switching Signals
TX_EN RX_EN SW_RF (50W Load) Result
H L Loaded TX through TP1
L H Loaded RX through
TP1
H L Not Loaded TX through Antenna
L H Not Loaded RX through Antenna