Motorola 38C V100 Service Manual page 17

Level 3 Service Manual
6881036B20
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).
DMCS TX_KEY
goes high goes high
TX starts
Figure 6. Saturation Detect Signal Control of PA Drive
MOTOROLA CONFIDENTIAL - PROPRIETARY
November 20, 2000
SAT_DET SAT_DET
goes low goes high
Linear ramp Ramp down
down begins ceases
Circuit Description
TX_KEY DMCS
goes low goes low
000718-O
13