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c. Digital-to-Analog (D/A) Converter
(U200)
The
digital-to-analog (D/A) converter, U200, is
a
multifunction
CMOS
integrated circuit containing two
7-bit D/A converters, one 4-bit D/A converter, six con-
trol
outputs, two
SPDT
transmission
gates,
and a
microcomputer interface.
The
output
(U200,
pin 11) of
the first 7-bit D/A con-
verter supplies the tuning voltage
for the
reference
oscillator, U301. When the
R/T
line is low
(OV), the
output of the
second 7-bit D/A converter is
routed,
via
an internal switch, to
pin 9.
This
provides the power
control reference
voltage
for the
TX
ALC
IC, U201,
during transmit operation.
In
vhf radios
only, when the
R/T
line is high
(5V),
the
second D/A converter's
output is
switched
to pin 8,
providing tuning
voltage
for the vhf 2-pole
filter,
U1.
A
combination
of
resistors R218
and
R219,
and a
microcomputer-interface-controlled switch (connected
internally between pins 15 and
16 of
U200) allows
extension
of the
2-pole tuning voltage range beyond
that of the 7-bit D/A converter.
In
mid-band and uhf radios there is
no
receive
tun-
ing adjustment,
so
the
second 7-bit D/A converter is
used only
for transmit.
The
4-bit D/A converter is
not
used
in
SABER
radios,
but its four pull-down
resistors are used.
These
resistors, which connect internally
to
U200, pins 4
through 7, are connected externally to the
BCD
fre-
quency switch, $823, and U400.
Three
of
U200's
control outputs are used
in
SABER
radios:
e
Pin 2 is the
REMOTE ANTENNA ENABLE
line;
a
high output
on
this
line
enables
the remote
antenna.
e
Pin
3
is the low-power range
enable
line (normally
low); a high
on
this line enables the very-low
power tuning range.
e
Pin 20 is the
clock
shifter enable line;
a
low on this
line
enables
the
clock
shifter.
d.
Antenna Switch (U201,
U203)
(1)
Mid-Band,
VHF
When the
PTT
switch is pressed, or the
OPTION
SELECT
line
is brought to 0
Vdc,
the microcomputer
(U400) sends data
to
U101, which sets the
R/T
line
low (OV).
A
logic low
on
U201,
pin
9,
causes U201,
pin
28
to
go
high
(6Vdc).
This voltage
is applied to the
anode side
of
a series-connected
pair of PIN
diodes,
internal
to
U203 (pin
8),
which control the
transmit/receive
rf
steering.
The
cathode side
of
the
diode pair is connected
to
U203,
pin 9.
During transmit operation, the PIN diodes are for-
ward biased and a low-impedance path connects
U203,
pin 1, to
the
selected antenna. When biased
for
transmit operation, the
voltage dropped between pins
8
(+)
and 9 of U203 should be two diode drops or
approximately 1.5 volts.
During receive operation, the
R/T
line
goes
high
(5V),
and
U201,
pin
28 (anode
bias),
should go
to
approximately
0Vdc. U201,
pin
26 (cathode
bias),
should pull-up
to
approximately 7.5V, reverse-biasing
the
T/R
PIN diode pair, resulting
in
a
low-impedance
rf
path from
U203,
pin
10, to the
selected antenna.
The
standard/remote antenna switch position is
determined by the
voltage
on the
OPTION
SELECT
line
(U400,
pin
62). When the OPTION
SELECT
line is
at
5V
or
1.24V, the microcomputer commands U200
to
bring the
REMOTE ANT ENABLE
line
(U201,
pin
23)
low (OV),
selecting the standard antenna. When U201,
pin
23, is
low,
U201,
pin
24,
is
also
low,
and U201,
pin
20,
is high
(7.5V). This reverse-biases
the PIN diode
pair that makes
up
the standard/remote antenna
switch
in
U203 (U201,
pin
24,
is the
anode; U201,
pin
20, is the cathode). When the diodes are reverse-
biased,
a
low-impedance
rf
path
exists between U203,
pin 14
(standard antenna) and the transmitter
or
receiver. Additional filtering is provided
in
vhf radios by
capacitors C206,
C207,
and
C208,
and inductor
L201,
and
in
mid-band radios by
capacitors
C207
and
C208,
and inductor L201.
Setting the
OPTION
SELECT
line to
3.74V
or
2.5V
causes
the microcomputer
to
instruct U200 to bring
the
REMOTE ANT ENABLE
line high
(5V). This caus-
es
U201,
pin
24,
to
go
high and
U201,
pin
20,
to
go
low,
forward-biasing U203's standard/remote antenna
switch PIN diodes, and forming
a low-impedance
path
from
U203,
pin 12, to the
receiver
or transmitter.
When the
PIN
diodes are forward-biased, the volt-
age
dropped between pins 12
(+)
and 13 of
U203
should be two diode drops or approximately 1.5 volts.
In
vhf radios,
capacitors C222, C223, C224,
and
C225,
and inductor L205 are for
rf
decoupling;
C229
is
a
de block and C241 is
a
matching element.
In
mid-
band radios, capacitors
C222, C223,
and
C224,
and
inductor
L205
are
for
rf
decoupling;
C229, C231,
and
L206
provide additional filtering on the remote antenna
path. Mid-band radios
also have a reverse-bias
circuit,
consisting
of
capacitor C250, inductors L250
and
L254, and diode
CR250. This
circuit prevents the PIN
diodes
from turning on
and starting to generate har-
monics at critical levels.
Proper operation of bias circuits
in
U201 is
depen-
dent on correct voltages being present on the
TX
5V
and
RX
5V
regulators, and resistors R211 through
R213.
Proper operation of U203 is dependent
on
cor-
rect installation of the 4205577Q01 grounding clip.
(2)
UHF
Although the filter/detector/switch module is func-
tionally equivalent
in
both
vhf and
uhf
radios, the
elec-
trical realization of the two 4-port PIN diode
rf
switches
are somewhat different, and require slightly different
biasing circuits.
As
in
the vhf models, the
TX/RX
antenna switch-
ing
is controlled by the
R/T
line
(U201,
pin
9). When
the
R/T
line
is high
(5V),
the
RX
5V
regulator
in
U201
is on and supplying current
to
receiver U2.
The
supply
9
c. Digital-to-Analog (D/A) Converter
(U200)
The
digital-to-analog (D/A) converter, U200, is
a
multifunction
CMOS
integrated circuit containing two
7-bit D/A converters, one 4-bit D/A converter, six con-
trol
outputs, two
SPDT
transmission
gates,
and a
microcomputer interface.
The
output
(U200,
pin 11) of
the first 7-bit D/A con-
verter supplies the tuning voltage
for the
reference
oscillator, U301. When the
R/T
line is low
(OV), the
output of the
second 7-bit D/A converter is
routed,
via
an internal switch, to
pin 9.
This
provides the power
control reference
voltage
for the
TX
ALC
IC, U201,
during transmit operation.
In
vhf radios
only, when the
R/T
line is high
(5V),
the
second D/A converter's
output is
switched
to pin 8,
providing tuning
voltage
for the vhf 2-pole
filter,
U1.
A
combination
of
resistors R218
and
R219,
and a
microcomputer-interface-controlled switch (connected
internally between pins 15 and
16 of
U200) allows
extension
of the
2-pole tuning voltage range beyond
that of the 7-bit D/A converter.
In
mid-band and uhf radios there is
no
receive
tun-
ing adjustment,
so
the
second 7-bit D/A converter is
used only
for transmit.
The
4-bit D/A converter is
not
used
in
SABER
radios,
but its four pull-down
resistors are used.
These
resistors, which connect internally
to
U200, pins 4
through 7, are connected externally to the
BCD
fre-
quency switch, $823, and U400.
Three
of
U200's
control outputs are used
in
SABER
radios:
e
Pin 2 is the
REMOTE ANTENNA ENABLE
line;
a
high output
on
this
line
enables
the remote
antenna.
e
Pin
3
is the low-power range
enable
line (normally
low); a high
on
this line enables the very-low
power tuning range.
e
Pin 20 is the
clock
shifter enable line;
a
low on this
line
enables
the
clock
shifter.
d.
Antenna Switch (U201,
U203)
(1)
Mid-Band,
VHF
When the
PTT
switch is pressed, or the
OPTION
SELECT
line
is brought to 0
Vdc,
the microcomputer
(U400) sends data
to
U101, which sets the
R/T
line
low (OV).
A
logic low
on
U201,
pin
9,
causes U201,
pin
28
to
go
high
(6Vdc).
This voltage
is applied to the
anode side
of
a series-connected
pair of PIN
diodes,
internal
to
U203 (pin
8),
which control the
transmit/receive
rf
steering.
The
cathode side
of
the
diode pair is connected
to
U203,
pin 9.
During transmit operation, the PIN diodes are for-
ward biased and a low-impedance path connects
U203,
pin 1, to
the
selected antenna. When biased
for
transmit operation, the
voltage dropped between pins
8
(+)
and 9 of U203 should be two diode drops or
approximately 1.5 volts.
During receive operation, the
R/T
line
goes
high
(5V),
and
U201,
pin
28 (anode
bias),
should go
to
approximately
0Vdc. U201,
pin
26 (cathode
bias),
should pull-up
to
approximately 7.5V, reverse-biasing
the
T/R
PIN diode pair, resulting
in
a
low-impedance
rf
path from
U203,
pin
10, to the
selected antenna.
The
standard/remote antenna switch position is
determined by the
voltage
on the
OPTION
SELECT
line
(U400,
pin
62). When the OPTION
SELECT
line is
at
5V
or
1.24V, the microcomputer commands U200
to
bring the
REMOTE ANT ENABLE
line
(U201,
pin
23)
low (OV),
selecting the standard antenna. When U201,
pin
23, is
low,
U201,
pin
24,
is
also
low,
and U201,
pin
20,
is high
(7.5V). This reverse-biases
the PIN diode
pair that makes
up
the standard/remote antenna
switch
in
U203 (U201,
pin
24,
is the
anode; U201,
pin
20, is the cathode). When the diodes are reverse-
biased,
a
low-impedance
rf
path
exists between U203,
pin 14
(standard antenna) and the transmitter
or
receiver. Additional filtering is provided
in
vhf radios by
capacitors C206,
C207,
and
C208,
and inductor
L201,
and
in
mid-band radios by
capacitors
C207
and
C208,
and inductor L201.
Setting the
OPTION
SELECT
line to
3.74V
or
2.5V
causes
the microcomputer
to
instruct U200 to bring
the
REMOTE ANT ENABLE
line high
(5V). This caus-
es
U201,
pin
24,
to
go
high and
U201,
pin
20,
to
go
low,
forward-biasing U203's standard/remote antenna
switch PIN diodes, and forming
a low-impedance
path
from
U203,
pin 12, to the
receiver
or transmitter.
When the
PIN
diodes are forward-biased, the volt-
age
dropped between pins 12
(+)
and 13 of
U203
should be two diode drops or approximately 1.5 volts.
In
vhf radios,
capacitors C222, C223, C224,
and
C225,
and inductor L205 are for
rf
decoupling;
C229
is
a
de block and C241 is
a
matching element.
In
mid-
band radios, capacitors
C222, C223,
and
C224,
and
inductor
L205
are
for
rf
decoupling;
C229, C231,
and
L206
provide additional filtering on the remote antenna
path. Mid-band radios
also have a reverse-bias
circuit,
consisting
of
capacitor C250, inductors L250
and
L254, and diode
CR250. This
circuit prevents the PIN
diodes
from turning on
and starting to generate har-
monics at critical levels.
Proper operation of bias circuits
in
U201 is
depen-
dent on correct voltages being present on the
TX
5V
and
RX
5V
regulators, and resistors R211 through
R213.
Proper operation of U203 is dependent
on
cor-
rect installation of the 4205577Q01 grounding clip.
(2)
UHF
Although the filter/detector/switch module is func-
tionally equivalent
in
both
vhf and
uhf
radios, the
elec-
trical realization of the two 4-port PIN diode
rf
switches
are somewhat different, and require slightly different
biasing circuits.
As
in
the vhf models, the
TX/RX
antenna switch-
ing
is controlled by the
R/T
line
(U201,
pin
9). When
the
R/T
line
is high
(5V),
the
RX
5V
regulator
in
U201
is on and supplying current
to
receiver U2.
The
supply
9
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