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Squelch Circuit - TM-11-5820-222-350060
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TM-11-5820-222-35 Radio Sets AN/VRC-24 and AN/TRC-68 Manual
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Input Circuit


Section II. DIRECTIONAL COUPLER AND RF INPUT STAGES
and the transmission line is ar-
10. Directional Coupler
ranged so that the effect of capaci-
tively coupled current flowing to-
a. General.
ward resistor R1306 is cancelled
(1) Transmitted and received rf sig-
by the inductively coupled current
nals travel to and from the antenna
flowing in the opposite direction.
on the transmission line through
However, the capacitively coupled
the directional coupler subunit.
current flowing in the direction of
The directional coupler subunit
CR1301 tends to reinforce the in-
consists of two directional coupler
ductively coupled current flowing
circuits, which use short trans-
in that direction. Thus, the incident
mission lines for coupling ele-
waves induce a resultant current
ments. The directional coupler
in coupling element A, which flows
circuit which includes coupling ele-
toward diode CR1301.
ment A is arranged to sample the
(3) Reflected waves, traveling along
incident waves of transmitter
the transmission line from the an-
power (traveling toward the an-
tenna, also induce a resultant cur-
tenna) and provide a front panel
rent flow in coupling element A be-
meter indication of output power
c a u s e of the inductive and
(PWR).
capacitive coupling between coup-
(2) The directional coupler circuit
ling element A and the transmis-
which includes coupling element B
sion line. Because of the polarity
is arranged to sample the reflected
reversal of reflected waves, the
waves of transmitter power (trav-
resultant current flow induced i
eling toward the rf and power am-
coupling element A is in the op-
plifier subunit) and provide a front
posite direction.
panel meter indication of the re-
(4) The resultant current flow is ab-
flected power (SWR). The metering
sorbed by resistor R1306, which
cirucits are shown in figure 22.
terminates coupling element A in
b. Directional Coupler Theory.
its characteristic impedance (51.1
(1) Directional coupler circuits A and
ohms). Thus, incident waves on the
B are identical except for reference
transmission line induce a result-
symbols, and they operate on the
ant current flow in coupling ele-
same principle. Only directional
ment A which is not affected by re-
coupler circuit A is discussed in
flected waves on the transmission
detail. The transmission line is
line. Diode CR1301 rectifies the
coupled to directional coupler A by
resultant  rf  current  induced
mutual inductance and capacitive
in coupling element A. Capacitor
coupling. Assume that the incident
C1301 filters the output of diode
waves induce a current to coupling
CR1301; resistor R1302 is the
element A which flows toward diode
diode load resistor
CR1301. Because of capacitive
coupling between the transmission
(5) The magnitude of the resultant
line and coupling element A, the
current is determined by the in-
incident waves will cause two equal
ductive and capacitive coupling be-
capacitive currents to flow in op-
tween coupling element A and the
posite directions in coupling ele-
transmission line, which is, in
ment A; one toward diode CR1301
turn, determined by the operating
and one toward resistor R1306.
frequency of the radio set. There
fore, the other end of coupling ele
(2) The inductive and capacitive coup-
ling between coupling element A
ment A is terminated by capacitor


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