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The microphone and balanced audio signals are summed and limited by U3A. Op-Amp U3B provides
audio level temperature compensation (for synthesizer sensitivity variations). Due to the many
different characteristics of various synthesizers, many components are selected for best performance
over the temperature range.
The audio is then filtered by a 10th order linear phase low pass splatter filter U3a and U3B (see
Figure 4). This is to provide the linearity and cut-off attenuation response required for narrow band
operation and digital applications.
The filter output level is set by R29, the deviation control. From this point, the signal goes to U5 a
bilateral audio switch. U5 is normally configured to be always on. The output of U5 goes to the final
buffer amplifier U3D. Installing JU23 lowers the audio level by one half facilitating a quick conversion
of a wide band transmitter for narrow band operation. The output of U3D has many capacitors with
values selected depending on the installed synthesizer module type.
4.0V
LIMITE R
+8.0V SWD
R84
10K0
C50
4.7uF
C17
10nF
a
U3
4
MC33174
R17
3
SEL
1
2
11
R18
3
SEL
2
JU24
R16
100K
1
1 2
TP10
C43
150pF
Limiter and Splatter Filter
4.0V
TE MP E RATURE
CO MP E NS ATIO N
R85
10K0
C49
10nF
TP5
U3
b
MC33174
R27
5
SEL
7
1
2
6
R19
R20
R23
Y
SEL
SEL
SEL
1
2
R28
X
SEL
R25
R24
SEL
X
1
N/I
TP14
2
JU3
Y
R26
3
N/I
JU7
Y
X
1 0 P O LE LO WP AS S FI LTE R
2
3
1
P11
+8.0V SWD
JU22
U4
LTC1569-6
P9
P10
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
MT-3 FM Transmitter Mainboard Instruction Manual
19
Theory of Operation
+8.0V SWD
1
C6
1.0uF
R66
R67
2
22K1
30K9
JU6
B0314
FIGURE 4
IM20-MT3TXMN
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