Motorola Internal Use
5
V2288 – Circuit Description
12)
The Loop filter basically acts then as a huge capacitor and resistor to give a long CR
time for smoothing. It uses a small capacitor and the very high input impedance
buffer Op-Amp. During the
TX_EN
(Whitecap) period when the transmitter is
preparing to operate the capacitor charges, then on receipt of
DM_CS
(Whitecap)
when the Transmitter actually fires; the capacitor discharges through the Op-Amp
giving a smooth tuning voltage, carrying modulation to the TX VCO. The support
voltage for the Loop filter is
V1_SW
(
V1
from GCAP II through Q913).
13)
The TX VCO IC now creates our required output frequency with the support signals
DCS_TX_VCO
(
DCS_SEL
(U110)
+
TX VCO_EN
(Q140)
through
Q130
) and
GSM_TX_VCO
(
GSM_SEL
(U110)
+
TX VCO_EN
(Q140)
through
Q130
), to
enable either GSM or DCS frequency production and the IC Power is supported by
SF_OUT
(MAGIC).
14)
The signal is then fed out through a buffer amplifier
Q320
, which is switched on and
off via
DM_CS
and supported by
RF_V2
(MAGIC)
15)
To prevent the output frequency from the TX VCO before stabilisation has occurred,
being amplified and transmitted, there is an
Isolation Diode CR320
placed. This is
biased ‘on’ by the exciter voltage from the
PAC IC U350
(Power Amplifier Control
IC); this allows the TX output frequency through to the
Exciter Amplifier Q330
and
at same time gives more or less drive to the exciter stage.
16)
The signal is then fed to a two stage, wide bandwidth
PA
made up from
Q331
and
Q370
, these are driven by the exciter voltage from the PAC IC, and supported by
B+
and
REG_B+
(
Q332 / Q333
B+
regulated by
TX_EN
).
17)
PA matching is provided using the signal
TX_GSM_*DCS
(
TXVCO_EN
+
DCS_SEL
through
Q160
) to switch on or off the diodes
CR380 / 370 / 390 / 350
and
340
to match the PA between GSM and DCS using the inductive strips on the PCB.
18)
The amplified signal is then fed back to the
RF switch U100
, as discussed in
Receive
,
then passed to the
Ant / Aux Switch J300
and transmitted through either the
antenna
A100
or the Aux testing port.
RF:
Power Control Operation
1)
The
PAC IC U350
(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
Q370
.
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.
5)
EXC
Pin 7
.This output drives the power control port of the exciter. An increase of
this voltage will cause the exciter to increase its output power.
