Audio Limiter And Automatic Modulation Control; Audio Limiter; Automatic Modulation Control; Automatic Limiter Circuit - Realistic TRC-449 Service Manual

1. AUDIO LIMITER AND AUTOMATIC MODU-
LATION CONTROL
The modu lation control for th is model consists of
2 loops of AMC which detect a signal from the
output of the audio limiter (mic preamp) and from
the audio power amp and feed it back to limiter
diodes.
TR20
AF AMP LIMITER
Low pass filter
r
TR26
AM .TX.
021 022
MIC AMP
R84
I--~~'-----o MIC
Vee
(MODULATION
SIGNAl)
TR18
LIMITER
(CLAMPING)
Vee
R104
TR23
AMC
SWITCHING
023
BIAS SWITCHING
~I
AF . PWR. AMP.
Fig. 1 Circuit Diagram of Audio Limiter and
Automatic Modulation Control
1-A. AUDIO LIMITER
The mic signal is applied to pin 2 of IC3. The signal
is picked up at Pin 6 and detected by D20
(negative part of waveform). This current flows
into the base of TR 19, effectively decreasing it's
C-E impedance; which increases the Vcc applied
to TR 18's base. And TR-18's C-E impedance drops
as its base current increases. This provides a
variable voltage divider at the input of I C3 (divid-
ing between R84 and TR 18), thus making it
possible to attenuate and control the modulation
signal right at the input to the mic gain stages.
t
w
>
W
...J
Mf-
U::J
-0..
f-
::J
o
MODULATION SIGNAL---.
Fig. 2 Audio Limiter Characteristics
1-8. AUTOMATIC MODULATION CONTROL
(AMC)
The output level of IC3 is already controlled by
the audio limiter. However, to control maximum
modulation more precisely and stably, the AMC
circuit which feeds back the output amplifier(lC4)
into the limiter diode(D21, D22) is provided. The
output of IC3 is subjected to impedance matching
by the AF AMP(TR20) through the low pass
filter( L 10, L 11) and TX(AM) switching(TR22).
The output of IC4 is subjected to impedance
matching by the output transformer(T1) and sup-
plied to the TX FINAL and DRIVER stage. This
modulation signal is detected at the secondary of
T1 by the bias switching diode(D23) and makes
collector current flow in TR23, AMC transistor. At
this time, a voltage drop is developed across the
resistor( R 1 04) wh ich causes collector voltage to
decrease and forward bias current of both D21 and
D22 is reduced. I n other words, the output voltage
of I C3 is cl ipped for both positive and negative.
However, the cl ipped waveform as shown in Fig.
2-D causes the band-width to increase, thus a low
pass fi Iter( L 10, L 11), wh ich has the characteristics
as shown in Fig. 3, is used.
-tH-
~ -J
Detected
ZERO _ _ _ _ _ _ _ _ by 023
Fig.2A Modulation Wave at
Antenna Terminal
Fig. 26 Output
at T-1
\ I \ I
I
c+Pin g level
Fig. 2C Limiter Output with Fig. 2D Limiter Output with
small input signal large input signal
Fig. 3 Low Pass Filter
Frequency
Response
ATT. Level (dB)
o
10
20
30
1 2.8
INPUT FREQUENCY (KHZ)
2. AUTOMATIC LIMITER CIRCUIT(ALC)
(SSB mode)
Both AMC and ALC circuits function for AM and
SSB. For SSB we have an additional circuit as
described here.
CT7
ALC
FT1 TR6
Crystal Filter 7.8 MHz AMP
013
SWITCHING
R45
.
~----Mr---o
Vee
To Antenna Transmitter Stage
Fig. 4 ALC circuit (abbreviated)
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