Realistic trc-449 Service Manual page 13

Vcc 0
TR26
AM. TX.
SWITCHING
IC4
T1 of
TR20
AF AMP LIMITER
D21 D22
M
MIC AMP
TR19
K
D20
Detector
0
Vcc
R84
MIC
(MODULATION
SIGNAL)
TR18
LIMITER
(CLAMPING)
Vcc
R104
TR23
AMC
Low pass filter
L10 L11
D23
BIAS SWITCHING
VR7
AMC
Vcc
FET5
ALC AMP R172
R24
D13
SWITCHING
R45
Vcc
TR42
D47
CT7
1 046
C155
ALC 1 .-
7t
FT1
Crystal
Filter
TR6
7.8 MHz AMP
To Antenna Transmitter Stage
1. AUDIO LIMITER AND AUTOMATIC MODU-
LATION CONTROL
The modulation control for this 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.
AF. PWR. AMP.
by the AF AMP(TR20) through the low pass
filter( L10, L11) 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(R104) which causes collector voltage to
decrease and forward bias current of both D21 and
D22 is reduced. In other words, the output voltage
of IC3 is clipped for both positive and negative.
However, the clipped waveform as shown in Fig.
2-D causes the band-width to increase, thus a low
pass filter( L10, L11), which has the characteristics
as shown in Fig. 3, is used.
Detected
by D23
ZERO
Fig. 1 Circuit Diagram of Audio Limiter and
Automatic Modulation Control
Fig. 2A Modulation Wave at
Antenna Terminal
Fig. 2B Output
at T-1
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 TR19, 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 IC3 (divid-
ing between R84 and TR18), thus making it
possible to attenuate and control the modulation
signal right at the input to the mic gain stages.
MODULATION SIGNAL
Fig. 2 Audio Limiter Characteristics
1-B.
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(IC4)
into the limiter diode(D21, D22) is provided. The
output of IC3 is subjected to impedance matching
LI \
Clipping level
Fig. 2C Limiter Output with Fig. 2D Limiter Output with
small input signal large input signal
ATT. Level (dB)
Fig. 3 Low Pass Filter
Frequency
Response
2.8
INPUT FREQUENCY (KHZ)
2. AUTOMATIC LIMITER CI RCUIT(ALC)
(SSB mode)
Both AMC and ALC circuits function for AM and
SSB. For SSB we have an additional circuit as
described here.
Fig. 4 ALC circuit (abbreviated)
0
10 -
20 -
30 -