Motorola Internal Use
7
P7389i – Circuit Description
Support Voltage being
SF_OUT
(
MAGIC DM
Pin C1)
14)
The signal is then fed out through a buffer amplifier
Q330
, which is supported by
TX275.
The signal is also split with a sample of the output frequency being directed
back to the
MAGIC DM IC
Pin A3
, for use within the TIC part of the MAGIC DM as
part of the TX Phase Locked Loop.
15)
To prevent the output frequency from the TX VCO before stabilisation has occurred,
being amplified and transmitted, there is an
Isolation Diode CR330
placed. This is
biased ‘on’ by the exciter voltage from the
PAC IC U390
(Power Amplifier Control
IC)
Pin 7
; this allows the TX output frequency through to the
Exciter Amplifier Q331
and at same time gives more or less drive to the exciter stage.
16)
The signal is then fed to a wide bandwidth
PA U300
, this is driven by the exciter
voltage from the PAC IC, and supported by a –ve biasing voltage created and timed
by
TX275
(
RF_V2 + TX_EN
through
Q120
),
Filtered –5V
(
-5V
) and
DM_CS
(
Whitecap U800
Pin E2
). Also supported by the voltage
PA B+
(
DM_CS + B+
through
Q380
)
17)
PA matching is provided using the signals
TX_GSM
(
TX275 + *GSM_SEL
through
Q1101
) and TX_DCS (
TX275 + *DCS_SEL
through
Q1101
) to switch on or off the
diodes
CR300
through
CR306
to match the PA between GSM and DCS / PCS using
the inductive strips on the PCB.
18)
The amplified signal is then fed back to the
RF switch U150
, as discussed in
Receive
,
then either transmitted through the
antenna A1
or the
Accessory Socket RF Port J600
Pin 2
RF:
Power Control Operation
1)
The
PAC IC U390
(Power Amplifier Control Integrated Circuit) controls the power
control of the transmitter. Below is a list of the main signals associated with the PAC
IC and their purposes.
2)
The RF detector (
RF_IN
Pin 2
) provides a DC level proportional to the peak RF
voltage out of the power amplifier, this is taken via an inductive strip from the output
of the PA
U300
.
3)
DET_SW
Pin 11
. This pin controls the variable gain stage connected between the RF
detector and the integrator. The gain of the variable stage will be unity when
DET_SW
is low and will be 3 when
DET_SW
is high (floating).
4)
TX_KEY
Pin 10
. This signal is used to ‘pre-charge’ the Exciter and P.A. and occurs
20
µ
S before the start of the transmit pulse.