Motorola TALKABOUT T2688, Service Manual

Page: 62 / 77

Topaz T2688/T200, T2988/T205 L3 Circuit Description
11
Motorola Proprietary Information
59. Power Control is achieved in the following way:
60. RAMP is connected to the Gain Control circuit U202 Pin 3 . This is the positive input
of the comparator. This is the same ideology as the AOC line in Whitecap products.
VBAT is connected to the negative comparator input on Pin 4 to compare changes on
the battery level that causes lower or higher gain of the PA
Output of the
Comparator U202 is the signal APC (Automatic Power Control) Pin 1
that drives the gain input of the PA U201 Pin 14 . This is fed through Q202, which is
implemented as a safety latch. In TX Mode, the right side of Q202 is biased on from
VBAT on Pin 4; this allows the APC voltage to vary the PA Drive.
In TX Background mode or RX mode, PA_ON is low, this shorts the left side of
R298 (VBAT) is shorted to Earth.
APC is also controlled in unison with the handset temperature. Thermister TR1
(which is supported by the voltage AVCC ) gives a voltage reading, which is
proportional to the current handset temperature. This is fed back to VEGA Pin 40 . As
the temperature goes up the APC line is backed down slightly.
61. Sensing the high current flow to the PA back stage, Pin 8, controls the PA power
saturation. The
Sense Resistor, R203 ( 0.033 ohm) is used to sense the voltage
variations on the VBAT line when the PA draws more or less current and is
connected to
Pin 4 of Q201 . The transistor is biased on using VBAT (Pin 5) and the
change in voltage over the sense resistor is throughput from Pin 4 to Pin 3. From here
it will vary the non-inverting input of U202 therefore changing the APC level.
62. Expected DC level at Pin 3 Q201 is 3.4V for PA off, 3.3V for Power step 15 and
3.2V for power step 5.
63. The second part of Q201 Pin 1 to 3 is used to inverse the TXON on signal as
TXVCO_OFF to switch Q603 .
If TXVCO_OFF is low the TX VCO is on.
64. From the PA, the RF is fed out to the RF Switch U501 on Pins 1 (GSM) GSMPA , or
Pin 11 DCSPA . The operation of the RF Switch is explained in Point 4.
65. The appropriate RF signal will then be fed out to either the Antenna ANT 1 or to the
Auxiliary RF connector U113 .
Synthesiser IC IC401
66. The synthesiser IC is responsible for outputting control signals to both the RX and
TX VCO’s. The IC is divided into an IF section
Pins 2 - 7 and an RF Sectio n Pins 8 –
12.
67. Before any outputs from the IC can be used a reference must be made available. For
this we use the system clock. This clock is a 13Mhz clock which is generated in the
13Mhz crystal oscillator, the tuning voltage is generated as the signal
AFC from the