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Voltage regulator U2A provides a regulated +4.0 VDC to all Op-Amp stages. Optional potentiometer
R14 can be installed (and JU15 removed) for special applications where a voltage other than
+4.0 VDC is required.
Data normally enters at P4-3, the Direct Modulation Input, and is connected via many possible
routes selected with jumpers. JU43 allows direct or on-board capacitor coupling. Op-Amp U2B
can be configured as a buffer with a DC offset to accommodate input data that has a positive DC
voltage offset. The data signals can be sent to gain buffers U2C and U2D through potentiometer
R48. Jumper JU14 is normally installed so that the output from U2D provides the correct data polarity
when using two-port modulation. See Figure 6.
A single TX Audio control line on P4-4 has 2 functions. It can be configured to switch U3D inputs
between data and voice. Alternately, it can be configured to remotely switch between wide band
and narrow band applications by switching between the regular audio path and one carrying half the
audio level as determined by resistive divider comprised of R22 and R21.
+9.5V CONTINUOUS
JU19
+8.0V SWD
1
3
2
X
Y
TP7
R55
49K9
B
JU26
Tx Audio Control
JU31
R72
0R0
1
DE V IATIO N
R29
CO NTRO L
2
50K0
3
R58
0R0
3
Y
JU21
4.0V
JU18
2
R79
X
R22
49K9
1
49K9
C
JU25
3
Y
Q1
R21
BC817-25
2
49K9
E
X
1
C55
R75
4.0V
100nF
10K0
TX Audio Control Circuitry
Y
X
1 0 P O LE LO WP AS S FI LTE R
2
3
1
+8.0V SWD
P11
JU22
U4
LTC1569-6
P10
P9
1
8
In+
Vout
R68
3K48
2
7
In-
V+
3
6
Gnd
Rx
R69
1
4
5
2K00
C21
V-
Div/Clk
1.0uF
2
P12
+9.5V CONTINUOUS
R86
4.0V
10K0
C24
10nF
C45
U3
d
4.7uF
MC33174
12
U5
14
TC4S66F
5
13
R37
1
2
49K9
R50
4
49K9
C44
JU23
330pF
3
R88
R34
R77
49K9
49K9
20K0
C23
JU34
330pF
1
3
X Y
- - - - - - - >
2
WIRE JUMPER
C28
FOR DUAL
4.7uF
R12
R35
4.0V
PORT
N/I
SEL
MODULATION
5
+8.0V SWD
ONLY
1
2
1
4
X
R80
2
JU35
3
27K4
U6
Y
TC4S66F
R83
3
3
50K0
TCXO DC
2
LE V E L
JU39
1
ADJUS T
MT-3 FM Transmitter Mainboard Instruction Manual
Theory of Operation
+8.0V SWD
1
C6
1.0uF
2
R66
R67
22K1
30K9
JU6
C40
JU29
1.0uF
C33
1.0uF
P3
JU8
TP8
1
R89
C46
JU32
100R
1.0uF
VOICE + SUBAUDIBLE
C47
1.0uF
C27
JU44
100nF
JU45
C56
33nF
1
2
TCXO MO DULATIO N
R36
TP9
50K0
LE V E L ADJUS T
JU38
2
C12
1.0nF
1
3
R53
Tx Audio Control
R32
N/I
SEL
R81
SEL
JU41
R78
10K0
c
U3
MC33174
C34
R54
1
9
R44
SEL
47uF
X
8
2
0R0
2
10
JU37
P4
Y
DCS, LTR
3
C41
R45
LOW FREQUENCY / DIRECT
10nF
N/I
MODULATION OUTPUT
B0316
FIGURE 6
IM20-MT3TXMN
21
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