N.b. Agc Circuit; Noise Blanker Circuit; Noise Amplifier Circuit - Realistic TRC-449 Service Manual

The SSB signal is processed through FT1, amplified
by the 7.8 MHz amplifier(TR6) and fed to the TX
stage. The peak-to-peak power output of the TX
stage is subjected to negative detection .by 046 an?
applied to the gate of FET5 ALC amplifier. At this
time, if the gate becomes negative, drain voltage is
increased up to about Vcc level. This increased
voltage is applied to the case of TR42 ALC
amplifier, and therefore, base current is
incr~as~d
which also increases collector current; this
in-
creases R45 voltage drop and decreases collector
voltage.
The base bias of TR6 is supplied from the collector
of TR42 through the switching diode(013). With a
decrease in the collector voltage of TR42, the base
bias current of TR6 js reduced and the operating
point is shifted, thus amplifier output level is
decreased as shown in Fig.5. This decrease will
limit the power output at the antenna terminal.
A L C ON condi t ion
Ope rating Poi n t
A LC 0 F F cond it ion
Operat ing Point
Input Signal
Input Signal
Fig. 5 Operating Characteristics of TR-6
3. NOISE BLANKER CIRCUIT
3-A. NOISE AMPLIFIER CIRCUIT
The Noise Blanker circuit is tuned to 23 MHz.
Any noise that exists in the 27 MHz frequency
range will also be present at 23 MHz. L 1 is tuned
to 23 M Hz, I C 1 ampl ifies the 23 M Hz band of
frequencies and L2 provides further tuning. This
signal is then amplified and detected (utilizing
~he
non-I inear characteristic portion of the operating
curve) by TR2, N.B. AMP. Also, the phase of the
signal . is reversed in TR2 and fed to the base of
TR3 for further amplification. Base current flows
when a pulse is applied; then the C-E impedance
is reduced to about 10-20 ohms. Since the col-
lector is connected to the hot side of L5 (RX 7.8
MHz), the primary side of L5 is in effect short-
circuited and so the pulse noise is eliminated from
the Receive signal.
J..B. N.B. AGC CIRCUIT
In some N.B. circuits, under conditions of high
signal and high noise, when N.B. is turned on,
there is more noise than with N.B. off. This is due
to a lack of AGC. To prevent this effect, a portion
of the 27 MHz signal is picked off of L4 and ap-
plied through FET601 and TR601, N.B. AGC
amplifiers. This signal is detected by 0601 and
0602 and applied through 0603 to the base of
TR 1, which drives the base into saturation. At the
same time, collector current increases, which drops
most of Vcc across R4; thus reducing the gain of
the N.B. Amplifier chain. This AGC effectively
controls the N.B. loop and its effect on the 27
MHz RF amplifier circuitry. N.B. AGe is set to
operate with an incoming 27 MHz signal level of
100-500
J1
V.
L1
ANTENNA
t
~i
lel
;;
<0
o
RX. Signal
/
-/
Pulse Noi ..
Fig. 6 Noise Blanker Circuit
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