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the channel selection knob. If channel change is in-
itiated by the A/B button, the tune line circuit is used to
<'1
initiate the tuning sequence. Refer to the paragraph that
.'
} describesthe tune line circuit.
5.3.5
Tune Power Detector
...
The tune power detector consists of U5A, U6A,
and U6B. A voltage proportional to the forward power
from the transmitter output is applied to U6-5. When
the forward power level is between 1 and 6 watts, U5A is
activated and a logic low is applied to the POWER ACK
port of the microcomputer to verify that the proper
power level for tuning is present.
5.3.6
Tune Line Circuit
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5.3.6.1
The tune line circuit consists of U5F, U5G,
U7E, U7F, and a portion of U9. During a tun-
ing sequence, U8-33 applies a low to U7-14. The high at
U7-15 activates U5G which applies a low to the antenna
tune (ANT TUNE) line. A low on the ANT TUNE line
enables the radio to key the transmitter on the selected
channel at a low power level (approximately 3 watts).
5.3.6.2
When the radio is in the receive mode, 3.3 V is
applied to the ANT TUNE line by the radio if
a B channel is being received. If an A channel is being
received, 12 V is applied to the ANT TUNE line. U5F is
activated by the 12 volts applied during A channel
reception; U5F is deactivated when 3.3 volts is applied
during B channel reception. The logic high at U8-33
produces a low at U7-12 that enables the transmission
gate in U9 to apply the output voltage ofU5F to U8-31.
The output of U5F is 0 V when an A channel is selected,
5 V when a B channel is selected.
5.3.6.3
When the tuner is not in a tuning sequence, a
transition in the voltage level at U8-31 is an in-
dication of a channel change from an A to a B channel
or vice versa. This initiates a tuning sequence for the
newly selected channel.
5.3.7
Voltage Control and Regulation
The voltage control and regulation circuit con-
sists of Q23, Q24, and U4. Final voltage regulation is
provided by U4.
5.3.8
Microcomputer
.
Microcomputer U8 controls the operation of the
automatic antenna tuner. All of the programs that con-
trol tuner operation are stored in the microcomputer on-
board memory.
5.4
SEQUENCE OF TUNER OPERATION
-
5.4.1
Power Applied
)
-,.:7
When dc power is applied to the SWITCHED
A + line (GRN), the microcomputer is reset.
5.4.2
Initialization and 4: 1 VSWR Detection
The microcomputer
applies a low to U8-33 to
produce an 0.4 volt signal at the ANT TUNE line. The
radio is enabled in the tune mode and generates an on-
channel signal at approximately 3 watts. If the VSWR is
greater than 4: 1, U6C applies a high to U8-39 and the
microcomputer begins the 4: 1 VSWR tuning sequence.
If the VSWR is less than or equal to 4: 1, U6C applies a
low to U8-39 and the tuner retains the tuning elements
in the current configuration until a channel change oc-
curs.
5.4.3
4: 1 VSWR Tuning Sequence
The microcomputer begins a sequence of switch-
ing tuning elements in and out of the rf line. During the
tuning sequence,
the microcomputer
monitors
the
voltage level at U8-1 and U8-39. When a 4:1 match is
achieved, a low is applied to U8-39. A list of tuning
elements '3lsed to achieve the 4: 1 VSWR is stored in the
microcomputer memory, and the microcomputer begins
the 2: 1 VSWR sequence. If a 4: 1 VSWR cannot be
achieved, the tuner reverts to the tuning configuration
previously stored in the microcomputer memory and
begins the termination of tuning sequence.
5.4.4
2:1 VSWR Tuning Sequence
U8-31 goes high to turn U5B on. If the VSWR is
greater than 2: 1, U6C applies a high to U8-39 and the
microcomputer begins a sequence of switching tuning
elements in and out of the rf line. (If the VSWR is less
than 2:1, the 1.5: VSWR tuning sequence is initiated.)
During
the
tuning
sequence,
the microcomputer
monitors the voltage level at U8-1 and U8-39. When a
2: 1 match is achieved, U6 applies a low to U8-39. A list
of tuning elements used to achieve the 2:1 VSWR is
stored
in the
microcomputer
memory,
and
the
microcomputer begins the 1.5:1 VSWR sequence. If a
2: 1 VSWR cannot be achieved, the tuner reverts to the
tuning configuration previously stored in the microcom-
puter memory and begins the termination of tuning se-
quence.
5.4.5
1.5:1 VSWR Tuning Sequence
U8-31 goes low to turn U5B off, and U8-32 goes
high to turn U5C on. If the VSWR is greater than 1.5:1,
U6C applies a high to U8-39 and the microcomputer
begins a sequence of switching tuning elements in and
out of the rf line. During the tuning sequence, the
microcomputer monitors the voltage level at U8-1 and
U8-39. When a 1.5:1 match is achieved, U6 applies a
low to U8-39. A list of tuning elements used to achieve
the 1.5:1 VSWR is stored in the microcomputer
memory, and the microcomputer begins the termination
of tuning sequence. U8-32 goes low to turn U5C off. If
a 1.5:1 VSWR cannot be achieved, the tuner reverts to
the tuning configuration
previously stored in the
5
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