MOTOROLA CONFIDENTIAL - PROPRIETARY
6881036B20
November 20, 2000
13
Level 3 Service Manual
Circuit Description
exciter amplifier
Q331
and, at same time, gives more or less drive to the exciter
stage.
The output of
Q331
is fed to wide bandwidth PA
U300
which is controlled by the
exciter voltage from the PAC IC.
U300
is
supported by a -ve biasing voltage created
and timed by TX275 (RF_V2 + TX_EN through
Q120
), filtered -5V (-5V) and DM_CS
(Whitecap IC
U800 Pin E2
).
U300
is also supported by the voltage PA B+ (DM_CS
+ B+ through
Q380
).
PA matching is controlled with TX_GSM (TX275 + GSM_SEL through
Q1101
) and
TX_DCS (TX275 + DCS_SEL through
Q1101
) which switches on or off diodes
CR300
through
CR306
to match the PA for GSM or DCS / PCS with the inductive
strips on the PCB.
The amplified signal is then fed to RF switch
U150
, as discussed in
Transceiver:
Receive
on Page 9. The amplified signal is either transmitted through antenna
A1
or routed to accessory socket RF port
J600 Pin 2
.
Power Control Operation
The Power Amplifier Control (PAC) IC
U390
controls the power output of the
transmitter. Below is a list of the main signals associated with the PAC IC and their
function.
• RF detector RF_IN (
U390
Pin 2
) provides a DC level proportional to the peak
RF voltage out of the power amplifier. RF_IN is taken via an inductively cou-
pled stripline at the output of the PA.
• DET_SW (
U390
Pin 11
) controls the variable gain stage connected between
the RF detector and the integrator. When DET_SW is low, stage gain is unity.
When DET_SW is high (floating), the gain is three.
• TX_KEY (
U390
Pin 10
) pre-charges the exciter and PA. It occurs 20ms before
the start of a transmit pulse.
• EXC (
U390
Pin 7
) drives the power control port of the exciter. Increasing this
voltage causes the exciter to increase output power.
• SAT_DET (
U390
Pin 12
). If the feedback signal from the RF detector lags too
far behind the AOC signal this output goes low indicating that the loop is at or
near saturation. SAT_DET signals the DSP to reduce the AOC_DRIVE signal
until SAT_DET rises (see Figure 6).
000718-O
Figure 6. Saturation Detect Signal Control of PA Drive
DMCS
goes high
TX starts
TX_KEY
goes high
SAT_DET
goes low
Linear ramp
down begins
SAT_DET
goes high
Ramp down
ceases
TX_KEY
goes low
DMCS
goes low
Содержание 38C V100
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