Theory of Operation
18
MT-3 FM Transmitter Mainboard Instruction Manual
IM20-MT3TXMN
fm audio Processor Signal Paths
Voice band audio normally enters the Balanced Input at P1-1 and P1-2 on the subrack connector
while microphone audio enters at P1-3. Potentiometer R31 sets the balanced compression level of
U1B while R2 sets the microphone compression level of U1A (see Figure 3).
Each amplifier has a dynamic range of 25 dB. Jumper JU17 is only installed when using very low
input levels (-18 to -25 dBm) and allows better tuning range for R31. The output of the compression
amplifiers are normally set for pre-emphasis (6 dB/Octave) but can be set for a flat audio response
using jumpers JU1 and JU2.
BALANCED
INPUT LEVEL
CONTROL
FLAT AUDIO
PRE-EMPHASIS
MICROPHONE INPUT
LEVEL CONTROL
BALANCED INPUT
hi gh = voi ce
l ow = dat a
X
Y
+9.5V CONTINUOUS
NOT INSTALLED FOR DIGITAL MOD.
AUXILIARY INPUT
LEVEL CONTROL
X
Y
MICROPHONE AGC PREAMP
BALANCED AGC PREAMP
hi gh = w i deband sel ect
X
Y
BALANCED
COMPRESSION
CONTROL
FLAT AUDIO
PRE-EMPHASIS
MICROPHONE
COMPRESSION
CONTROL
X
Y
X
Y
X
Y
FLAT AUDIO
PRE-EMPHASIS
X
Y
TWO PORT POLARITY
SUBTONE 1 INPUT
CONTROL LEVEL
3
P1
2
P1
1
P1
4
P1
3
P3
3
P2
1
P2
4
P2
+9.5V CONTINUOUS
C10
10nF
4K53
R46
C5
10nF
1
2
3
R2
5K0
FB1
+8.0V SWD
+8.0V SWD
1
2
3
JU5
47uF
C13
3
2
1
100K0
R48
680nF
C7
4.7uF
C8
TP3
SEL
R57
N/I (22nF)
C32
Rect In
Var G
Rect Cap
Output
Inv In
R3
THD
Trim
15
14
16
10
12
11
9
U1
SA571D
a
680nF
C42
TP12
4.7uF
C26
Vcc
Rect In
Var G
Rect Cap
Output
Inv In
R3
THD
Trim
GND
2
3
1
7
5
6
8
13
4
SA571D
U1b
2
P2
JU16
9
10
8
U2
MC33174
c
R74
N/I
R73
N/I
13
12
14
U2
MC33174
d
4.0V
TP6
R43
11K8
C39
680nF
3
2
1
5K0
R42
C35
1.0uF
C1
1.0uF
C9
1.0uF
C30
1.0uF
R82
10K0
C37
3.3nF
R49
16k2
C14
10nF
4.0V
R76
16k2
C16
1.0uF
JU33
C51
1.0uF
1
2
3
JU12
R39
15K4
R41
137K
R59
20K0
R62
15K4
R5
1M00
C25
1.0uF
R60
18K2
R61
18K2
R8
2K21
R9
49K9
R4
18K2
R6
18K2
330nF
C36
C15
3.3nF
R7
2K21
R10
49K9
330nF
C19
C2
1.0uF
R51
10K0
2
1
3
JU1
2
1
3
JU2
JU11
TP11
TP4
JU10
R52
100K
R65
100K
C38
3.3nF
R11
2K21
R40
49K9
2
1
3
JU14
1
2
3
R31
5K0
R30
604R
TP2
0R0
R1
R64
100K
R56
0R0
C4
1.0uF
2
3
1
JU36
JU17
1
2
3
JU9
1
2
3
5K0
R63
3
2
1
5K0
R38
MICROPHONE INPUT
SUBTONE INPUT 1
TONE/DIGITAL I/P
Microphone and Balanced Input Circuitry
B0313
FIGURE 3
