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Figure 1. Receiving signal path, simplified block diagram.
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TM-11-5820-222-35 Radio Sets AN/VRC-24 and AN/TRC-68 Manual
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Squelch Circuit - TM-11-5820-222-350060


RECEIVER THEORY
Section I. RECEIVING SIGNAL PATH
receiver mixer V104 to produce the
8. Receiver Rf Sections
20.0- to 29.9-mc first if. After
(fig. 3)
amplification by first and second
The signal path for the receive function
if. amplifiers V301 and V302, the
of the rt unit is shown in block diagram
first if. signal is coupled to second
form in figure 3. The block diagram also
receiver mixer V303, where it is
shows the positions of transmit-receive
heterodyned with the signal from
(tr) relay contacts in the signal path.
second oscillator V305 to produce
a. General. The rf unit operates as a
the 3.0- to 3.9-mc if. signal.
receiver at all times when in the standby
(2) The second oscillator uses 10
condition. On receive, the tr relays are
crystals covering the 17.0- to 26.0-
deenergized because the push-to-talk
mc range in 1.0-mc steps. The
switch on the transmitter microphone is
output of V303 is the difference
not operated.
frequency of the two inputs and is
b. Rf Section. The 225.0- to 399.9-mc
in the 3.0- to 3.9-mc frequency
input rf signal feeds from the uhf antenna
range.
through the directional coupler and through
(3) The 3.0- to 3.9-mc triple-tuned
antenna relay K101 to first rf amplifier
bandpass filter, consisting of fil-
V102. The rf signal is amplified by first
ters Z401, Z402, and Z403, passes
and second rf amplifiers V102 and V103.
the 3.0- to 3.9-mc if. signal and
The rf signal is coupled from the second
applies it to third receiver mixer
rf amplifier to first receiver mixer V104.
V401A. The 3.0- to 3.9-mc signal
The uhf injection subunit provides the
is heterodyned with the output of
heterodyning frequency to the first re-
third oscillator V401B to produce
ceiver mixer through injection relay K102.
the 500-kc if. signal.
c. Uhf Injection.
(4) The third oscillator is controlled
(1) The uhf injection system consists
by 10 crystals covering the 3.0- to
of first oscillator V201, frequency
3.9-mc range in 0.1-mc steps. The
tripler V202, and first, second, and
third oscillator output is 0.5 mc
third uhf injection amplifiers V203,
higher than the if. when the if. is
V204, and V205. The first oscil-
between 3.0 to 3.4 mc. The third
lator V201 provides 18 crystal-
oscillator output is 0.5 mc lower
controlled out put frequencies.
than the if. when the if. is between
Frequency tripler V202 selects the
3.5 and 3.9 mc. Thus, the output of
third harmonic of these frequen-
third receiver mixer V401A is al-
cies to produce 18 frequencies
ways 0.5 mc (500 kc). The 500-kc
which cover the 200- to 370-mc
if. is passed by 500-kc if. filter
range in 10-mc steps.
FL901 and is amplified by three
(2) The output of V202 is amplified by
500-kc if. amplifiers V501, V502,
three rf amplifiers, V203 through
and V503.
V205. The output of V205 is applied
to the first receiver mixer V104
9. Receiver Audio Section
through injection relay K102.
(fig. 3)
d. If. Section.
a. Audio Input Section.
(1) The uhf injection frequency and the
(1) The output of V503 is applied to
received rf are heterodyned in first


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