Table of Contents
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Initially,
an audio signal enters the enabled micro-
phone and the audio is routed
to
the audio preampiifi-
er,
U102
(pin
21
for internal
microphone;
pin
22 for
external microphone), where some
necessary shaping
and filtering is done. Next, the output (pin
11) of
U102
is fed through
capacitor
C23,
and resistors
R17
and
R18
(part of the pre-emphasis/limiter circuit) to pins
11
and 10 of audio
filter
U101.
Within U101, the
TX
filtering is enabled for flat
audio or pre-emphasis, and
PLIDPL
encode is set.
From the output of the
limiter,
the signal then
goes
through the splatter filter to the summer, where the
microphone input is summed
with
the
AUX
TX
input
and the
PL/DPL
encode signal. The
PL
tones are gen-
erated by U101, using the
PL
sample clock signal
(U400,
pin
39)
as a
reference.
This clock
signal is
a
square wave
multiple of the
desired
PL
frequency.
The
summer output then
goes
through
a
buffer into two
attenuators.
A
five-bit attenuator adjusts the
VCO
modulation
level, then sends the signal
(VCO
MOD)
from
U101,
pin
21
to
pin
3 of
synthesizer/VCO
module U300.
The
four-bit attenuator
adjusts the reference
modulation
level, then sends the signal
(REF
MOD)
from
U101,
pin
20,
to
U300,
pin 19.
(2) Transmit
RF
(U202)
The
frequency-modulated, on-channel signal
from
U300
(pin 14) is fed to pin
1
of the
rf
power amplifier
(PA),
U202.
The
level of this
input is nominally
+5dBm.
The
mid-band PA is a
3-stage
amplifier with
adjustable gain
in
the 1.0- to 6.0-watt range.
The
vhf
PA
is
a 3-stage
amplifier, with
adjustable gain
in
either
the 1.0- to 2.5-watt range or the 2.0-
to
6.0-watt range,
depending on the radio model.
The
uhf
PA
is
a 4-stage
amplifier, with
adjustable gain
in
either the 1.0- to 2.0-
watt range or the
2.0-
to
5.0-watt range, depending
on
the radio model. All
rf
power amplifiers have nominal
input
and output impedances
of
502.
In
the mid-band PA, the
second-stage collector
(U202,
pin
6) is used to control PA gain. The
third-
stage base bias
network
(U202,
pin
7) is also connect-
ed to
pin
6, providing
a variable base bias
to
maximize
efficiency at lower power levels.
The
third-stage col-
lector (pin 12) is tied directly to battery (unswitched)
B+.
The first-stage collector
(pin 3) is tied to
bias
switching transistor, Q204, which is turned
on only
in
transmit by the transmit automatic level control IC,
U201.
A
TX
5V
regulator
in
U201
supplies
+5V
to pin
16
(U201) only
during transmit.
A
switch within U201
causes
pin
17 to go low (OV), saturating
Q204; R209
is for current limiting.
When the
radio is not transmit-
ting, the
TX
5V
is low (OV) and
pin
17 is pulled up to
approximately +7.5V, which turns
off
Q204.
In
the
vhf PA, the first-stage collector (U202,
pin
3)
is used to control PA gain.
The second-
and third-
stage collectors
are tied directly
to
battery
(unswitched)
B+
(pins 6 and 12).
A
switching transis-
tor,
Q204, supplies base bias
to pin
7.
When the
TX
5V
regulator
in
transmit automatic level control IC,
U201, turns
on,
+5V
is supplied to
U201,
pin 16.
A
switch within U201
causes U201,
pin
17, to
go
low
(OV), saturating
Q204; R209
is
a
current limiting resis-
tor.
When
the
TX
5V
regulator is low (OV), U201, pin
17, pulls up
to
approximately
+7.5V
and
Q204
is
turned
off.
In
the
uhf PA,
the first-stage collector
(U202,
pin
2)
is supplied by
Q200,
which is connected to the
TX
5V
regulator (U201,
pin 16)
in
an emitter follower configu-
ration.
When the
TX
5V
regulator is on, regulated
+4.3V
is supplied
to
U202,
pin
2; when the
TX
5V
reg-
ulator is
off,
Q200
is cut
off
and no current
passes.
The second-stage
collector voltage (U202,
pin
3) is
used
to control
the gain of the uhf PA.
The
third- and
fourth-stage collectors (U202, pins 6 and 8) are tied to
battery
B+. Base bias
is supplied to
U202,
pin
4,
via
switching transistor
Q204 (PNP
Darlington).
The base
of
Q204
is tied to pin
26
of U201 through current-limit-
ing resistor
R209. When
the radio is receiving
(TX 5V
regulator off), U201,
pin
26,
is pulled up to
+7.5V,
turn-
ing off
Q204. When
the
TX
5V
regulator is on, the volt-
age
at U201,
pin
26 drops
to
approximately +4.5V, sat-
urating
Q204.
In
all radios
the gain control voltage for U202, pin
3, is supplied by U201
via pass
transistor Q202.
The
PA
control circuit inside U201
sets
the control
voltage
to
establish the correct
ratio
between the
RF
DET
volt-
age
from
the
FDS
module,
U203
(pin
5
mid-band and
vhf; pin 4 uhf), and the D/A reference
voltage
from
U200,
pin
9.
This
reference voltage is software con-
trolled and
depends
on
the current
channel's
pro-
grammed power level.
In
high-power model
PA
modules, an internal ther-
mistor is
connected between ground and U202
(pin
11
on
mid-band and vhf;
pin 9 on uhf).
Resistor R210
connects
the thermistor to the
TX
5V
regulator, form-
ing
a
voltage
divider.
The
resulting temperature
sense
voltage
is fed to pin
8
of
U201. Circuitry
within U201
causes
the
PA
power to cut
back (via
the control volt-
age supplied
to
U202,
pin
3)
if
the PA temperature
exceeds a preset value. The cutback
temperature is
determined by the
value
of
R210.
i.
COPE
Microcomputer (U502)
Refer
to
Figure 3 and
the
2k
and 8k schematic
diagrams
in the
applicable service manual.
The
control
of
peripheral electronics
(COPE)
microcomputer is the heart of the display board.
The
COPE
has several functions,
the main
ones being:
e
control of the liquid crystal display, which
displays
information about the
state
of the radio,
e
processing
of information input by the user
via
the
radio's keypad,
e
communication of channel information (stored
in
the
EEPROM)
to the
CORE
microcomputer,
giving
the radio expanded channel capability, and
e
control of the
DTMF
generator, U505.
13
Initially,
an audio signal enters the enabled micro-
phone and the audio is routed
to
the audio preampiifi-
er,
U102
(pin
21
for internal
microphone;
pin
22 for
external microphone), where some
necessary shaping
and filtering is done. Next, the output (pin
11) of
U102
is fed through
capacitor
C23,
and resistors
R17
and
R18
(part of the pre-emphasis/limiter circuit) to pins
11
and 10 of audio
filter
U101.
Within U101, the
TX
filtering is enabled for flat
audio or pre-emphasis, and
PLIDPL
encode is set.
From the output of the
limiter,
the signal then
goes
through the splatter filter to the summer, where the
microphone input is summed
with
the
AUX
TX
input
and the
PL/DPL
encode signal. The
PL
tones are gen-
erated by U101, using the
PL
sample clock signal
(U400,
pin
39)
as a
reference.
This clock
signal is
a
square wave
multiple of the
desired
PL
frequency.
The
summer output then
goes
through
a
buffer into two
attenuators.
A
five-bit attenuator adjusts the
VCO
modulation
level, then sends the signal
(VCO
MOD)
from
U101,
pin
21
to
pin
3 of
synthesizer/VCO
module U300.
The
four-bit attenuator
adjusts the reference
modulation
level, then sends the signal
(REF
MOD)
from
U101,
pin
20,
to
U300,
pin 19.
(2) Transmit
RF
(U202)
The
frequency-modulated, on-channel signal
from
U300
(pin 14) is fed to pin
1
of the
rf
power amplifier
(PA),
U202.
The
level of this
input is nominally
+5dBm.
The
mid-band PA is a
3-stage
amplifier with
adjustable gain
in
the 1.0- to 6.0-watt range.
The
vhf
PA
is
a 3-stage
amplifier, with
adjustable gain
in
either
the 1.0- to 2.5-watt range or the 2.0-
to
6.0-watt range,
depending on the radio model.
The
uhf
PA
is
a 4-stage
amplifier, with
adjustable gain
in
either the 1.0- to 2.0-
watt range or the
2.0-
to
5.0-watt range, depending
on
the radio model. All
rf
power amplifiers have nominal
input
and output impedances
of
502.
In
the mid-band PA, the
second-stage collector
(U202,
pin
6) is used to control PA gain. The
third-
stage base bias
network
(U202,
pin
7) is also connect-
ed to
pin
6, providing
a variable base bias
to
maximize
efficiency at lower power levels.
The
third-stage col-
lector (pin 12) is tied directly to battery (unswitched)
B+.
The first-stage collector
(pin 3) is tied to
bias
switching transistor, Q204, which is turned
on only
in
transmit by the transmit automatic level control IC,
U201.
A
TX
5V
regulator
in
U201
supplies
+5V
to pin
16
(U201) only
during transmit.
A
switch within U201
causes
pin
17 to go low (OV), saturating
Q204; R209
is for current limiting.
When the
radio is not transmit-
ting, the
TX
5V
is low (OV) and
pin
17 is pulled up to
approximately +7.5V, which turns
off
Q204.
In
the
vhf PA, the first-stage collector (U202,
pin
3)
is used to control PA gain.
The second-
and third-
stage collectors
are tied directly
to
battery
(unswitched)
B+
(pins 6 and 12).
A
switching transis-
tor,
Q204, supplies base bias
to pin
7.
When the
TX
5V
regulator
in
transmit automatic level control IC,
U201, turns
on,
+5V
is supplied to
U201,
pin 16.
A
switch within U201
causes U201,
pin
17, to
go
low
(OV), saturating
Q204; R209
is
a
current limiting resis-
tor.
When
the
TX
5V
regulator is low (OV), U201, pin
17, pulls up
to
approximately
+7.5V
and
Q204
is
turned
off.
In
the
uhf PA,
the first-stage collector
(U202,
pin
2)
is supplied by
Q200,
which is connected to the
TX
5V
regulator (U201,
pin 16)
in
an emitter follower configu-
ration.
When the
TX
5V
regulator is on, regulated
+4.3V
is supplied
to
U202,
pin
2; when the
TX
5V
reg-
ulator is
off,
Q200
is cut
off
and no current
passes.
The second-stage
collector voltage (U202,
pin
3) is
used
to control
the gain of the uhf PA.
The
third- and
fourth-stage collectors (U202, pins 6 and 8) are tied to
battery
B+. Base bias
is supplied to
U202,
pin
4,
via
switching transistor
Q204 (PNP
Darlington).
The base
of
Q204
is tied to pin
26
of U201 through current-limit-
ing resistor
R209. When
the radio is receiving
(TX 5V
regulator off), U201,
pin
26,
is pulled up to
+7.5V,
turn-
ing off
Q204. When
the
TX
5V
regulator is on, the volt-
age
at U201,
pin
26 drops
to
approximately +4.5V, sat-
urating
Q204.
In
all radios
the gain control voltage for U202, pin
3, is supplied by U201
via pass
transistor Q202.
The
PA
control circuit inside U201
sets
the control
voltage
to
establish the correct
ratio
between the
RF
DET
volt-
age
from
the
FDS
module,
U203
(pin
5
mid-band and
vhf; pin 4 uhf), and the D/A reference
voltage
from
U200,
pin
9.
This
reference voltage is software con-
trolled and
depends
on
the current
channel's
pro-
grammed power level.
In
high-power model
PA
modules, an internal ther-
mistor is
connected between ground and U202
(pin
11
on
mid-band and vhf;
pin 9 on uhf).
Resistor R210
connects
the thermistor to the
TX
5V
regulator, form-
ing
a
voltage
divider.
The
resulting temperature
sense
voltage
is fed to pin
8
of
U201. Circuitry
within U201
causes
the
PA
power to cut
back (via
the control volt-
age supplied
to
U202,
pin
3)
if
the PA temperature
exceeds a preset value. The cutback
temperature is
determined by the
value
of
R210.
i.
COPE
Microcomputer (U502)
Refer
to
Figure 3 and
the
2k
and 8k schematic
diagrams
in the
applicable service manual.
The
control
of
peripheral electronics
(COPE)
microcomputer is the heart of the display board.
The
COPE
has several functions,
the main
ones being:
e
control of the liquid crystal display, which
displays
information about the
state
of the radio,
e
processing
of information input by the user
via
the
radio's keypad,
e
communication of channel information (stored
in
the
EEPROM)
to the
CORE
microcomputer,
giving
the radio expanded channel capability, and
e
control of the
DTMF
generator, U505.
13
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