Theory of Operation
20
MT-3 FM Transmitter Mainboard Instruction Manual
IM20-MT3TXMN
If no compression circuitry is required, the balanced input signals can be routed around the
compression circuitry using jumper JU11. Op-Amp U2C then provides buffering and amplification.
Pre-emphasis filtering at the output is enabled or disabled using jumper JU9. The audio signal is then
normally routed to U3A, the summing amplifier / limiter and processed as indicated previously.
NOTE
: If compression is bypassed (see Figure 5), the THD may be higher due to the reduced
dynamic input range causing more clipping. “Key down” current consumption can be reduced
by 9 mA by disconnecting the compression amplifier power (removing JU36), however, the
microphone circuit will be disabled.
Assuming no data modulation, a second way to route around audio compression is via the direct
modulation port P4-3 and route through JU28 to R48 and Op-Amp U2C. Sub-audible tones enter
at P2-2 where level is controlled by R42. A single jumper JU16 enables the audio path to U3A, the
summing amplifier / limiter.
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
l ow = nar r ow band 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
P4
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
Bypassing Compression
B0315
FIGURE 5
