CIRCUIT DESCRIPTION
5-22
The bias current of Q503 is fixed at a constant
level by Q502. The collector current of Q503 flows
through R511, and the voltage drop across that resistor
(and therefore the current) is set by R508 and R509.
For example, if current through R2511 attempts to
increase, the emitter voltage of Q502 decreases. Q502
then conducts less and turns Q503 off slightly to main-
tain a constant bias current. This provides a stable bias
over changes in temperature. The output signal of
Q503 is applied to a 3 dB, 50-ohm pad formed by
R587-R589, and then coupled by C531 to antenna
switch U603. From U603 it is applied to the RF board.
5.11 DC POWER DISTRIBUTION
5.11.1 POWER ON OPERATION
When the On-Off/Volume knob is pressed to turn
power on (this is a push on/push off switch), the
following sequence of events occurs:
1. The power switch closes and grounds the emitter of
Q8 on the logic board.
2. If ignition switch sense is used, the 13V signal from
the ignition switch is applied to the base of Q8 and
pin A7 of microcontroller U6 (or pin 48 of micro-
controller U9 with the Rev 3 logic board). If ignition
sense is not used, pull-up resistor R145 can be
installed to make the transceiver functional.
3. Q8 then turns on which grounds the base of Q512 on
the PA board and turns it on. This turns on main
power switching transistor Q511 and applies power
to the switched portions of the transceiver.
5.11.2 POWER OFF OPERATION
When power is turned off, the following sequence
of events occur:
1. If the power switch is pressed, it opens and the base
of Q8 is no longer grounded. This also applies a high
signal to the microcontroller which then detects the
power-off condition.
2. If ignition switch control of power is used, turning
the ignition switch off causes the signal applied to
the base of Q8 to go low. This signal is also inverted
by Q5 and applied the microcontroller.
3. Q8 then turns off. However, when the controller
detects the power-down request, it holds Q2 on to
delay power turn-off until all the required save oper-
ations are complete.
4. The controller then turns off Q2 and both Q511 and
Q512 on the PA board turn off which turns off trans-
ceiver power.
5.12 LOGIC BOARD (VERSION C)
NOTE: The following describes the Version C logic
board (see Section 1.12.2). Version B is similar except
that the ADSIC has been replaced by the CODEC.
5.12.1 MICROCONTROLLER (U2)
Microcontroller U57 is PowerPC
®
based which
gives this transceiver processing power equal to some
current desktop computers. This microcontroller
provides all transceiver control functions except signal
processing which is provided by DSP U1.
Functions provided by U57 include detecting
button presses, processing incoming and outgoing
calls, displaying operational data to the user, and coor-
dinating control of the other processor (DSP).
The operating speed of U2 is controlled by
4.9152 MHz crystal Y2. The internal clock is five
times this frequency or 24.575 MHz (25 MHz
operational).
5.12.2 MEMORY
Memory devices include Flash, SRAM,
EEPROM, and DSP SRAM. There is 8 megabyte of
Flash that is used primarily for code storage but can
can also be used for as non-volatile memory.
The SRAM (static RAM) consists of four 256K x
16 devices (U20/U23/U71/U72). A 32K x 8 EEPROM
(U58) is used to store personality data.
5.12.3 DIGITAL SIGNAL PROCESSING
OVERVIEW
The Digital Signal Processing (DSP) functions
are performed by the DSP chip (U1) and the CODEC
(U2) with the support of microcontroller U57. Func-
DC POWER DISTRIBUTION
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