Kenwood TR-751A - SSB Squelch Circuit

TR-751A/E
CIRCUIT DESCRIPTION
The TR-751A/E AGC circuit has been designed to allow
the AGC voltage to control the GaAs FET similar to the
control that was obtained with the MOS FET. As shown
in Fig. 3, AGC voltage from an amplifier similar to that
used in previous models is fed into the intermediate fre-
quency amplifier. The AGC voltage is approx. 4V when no
signal is present. The AGC voltage is amplified by the
non-DC current inversion amplifier circuit that is com-
posed of Op Amplifier (1C3). Its output is then applied to
the GaAs FET. The output voltage is set to approx. 2.5V
when no signal is present, or at minimum RF gain. The
AGC characteristics are shown in Fig. 4. The AGC time
constant is automatically switched to slow in SSB mode
or to fast in CW mode. The high sensitivity of the receiver
system is thus obtained without sacrificing any two signal
characteristic.
Additionally, the RF gain control, provides a convenient
method of tuning out undesired signals even when receiving
signals that are too strong from near by local stations.
AGC
BUFFER OErELTOR
VF
AGC TIME CONSTANT
AGC AMP SWITCHING CIRCUIT
RF AMP GAIN
wav.
0d8 when G2 volta
G2 Voltage (v)
Fig. 4 AGC attenuation comparison
RF AMP AGC
VOLTAGE SHIFT CIRCUIT
VF VF
IF AMP AGC
1 r 71
0 +8V
THRESHOLO.
LEVEL AO. 3
2,03
3SK73 c20
SSB IF AMP O—=—| |
To
O RF AMP AGC LINE
NON-DC CURRENT
INVERSION AM
1C3 : NIMASSBC
g RF GAIN
CONTROL
fo)
Sse SOL TIME CONSTANT SWITCHING
Fig. 3 AGC circuit black diagram
©@ SSB squelch circuit
The TR-751A/E SSB suqlech circuit is a noise operated
type squelch. As compared with signal type squelch,
noise detection squelch may be opened even by very weak
signals, such as are frequently encountered in SSB.
The high sensitivity of the squelch circuit provides
advantages
when
receiving
VHF
signals
from
distant
sta-
tions
and
when
scanning.
Generally,
signal
type
squelch
cannot surpass noise detection type in sensitivity, since
they
are
opened
by
changes
in
the
AGC
voltage.
It
means
that, to open a signal type squelch, sufficient voltage level
of signal to deflect the $ meter is required. The sensitivity
of
the
TR-751A/E
squelch
is
0.14V
or
less
(a
weak
signal
which will not deflect the S meter.)
The squelch signal is applied to 1C2 used in FM mode,
through the SSB filter, SSB IF, and buffer amplifier. This
IC, mixes the signal with 10.24MHz to produce 455kHz.
Like the FM IF, the 455kHz signal is also amplified by 1C2
and applied to the same squelch circuit as that used in FM
mode. Since SSB signals do not contain carrier, unlike FM
signals, the time constant circuit is switched between FM
mode and SSB mode to get an appropriate response time.