T.O.
3112-4-101-1
Section
I
Paragraphs 1-30
to
1-35
benefit can be derived from the sharp, crystal passbands
of the 455-kc i-f crystal filter. The "XTAL PHASING
control (23) is part of the crystal filter. The 455
+
3-kc
bfo and associated "BEAT OSC." control (10) and the
"IF OUTPUT" connector
(6,
figure 2-2) are associ-
ated with the 455-kc i-f channel of the receiver.
1-30. A-F SECTION AND POWER SUPPLY. The a-f
section of the receiver incorporates an a-f amplifier to
develop the a-f power output of the receiver. The
"AUDIO GAIN" control (16, figure 3-1) is used to
control the receiver output to the "AUDIO OUTPUT"
terminals
(8,
figure 2-2) and that to the "PHONES"
jack (20, figure 3-1). The self-contained power supply
develops all the a-c and the d-c supply voltages used by
the receiver. The critical d-c supply voltages are regu-
lated to maintain constant the sensitivity and frequency
stability. The "RF GAIN" control (11) is part of the
power-supply bias rectifier. Its chosen setting, in con-
junction with the a-v-c system of the receiver, when
used, determines the gain developed in the r-f and i-f
sections of the receiver.
two microvolts for an A-M signal, and 0.75 microvolt for
a c-w signal, or better. The undistorted power output of
the receiver at threshold sensitivity is two watts or better.
This a-f output is provided at a signal-plus-noise to
noise power ratio of ten to one. This provides for
optimum receiver performance in the presence of a weak
signal tuned in, since the background noise caused by
the first electron tube and the antenna r-f tuner does
not interfere with reception. The receiver sensitivity may
be reduced in the presence of a strong signal by turning
the "RF GAIN" control (11, figure 3-1) counterclock-
wise from its maximum clockwise position. This is done
to preven; overloading of the r-f and i-f sections of the
receiver. The receiver a-f output may be decreased by
turning the "AUDIO GAINcontrol (16) counterclock-
wise. This is done to prevent overloading of the a-f
section of the receiver and to provide for-comfortable
reception at the desired a-f output level. When the re-
ceiver a-v-c system is operative and the "RF GAIN"
control is set -to its maximum clockwise setting, the re-
ceiver output remains constant within a one-to-four volt-
age ratio when the input is increased from 2 to 200,000
1-31.
CAPABlLlTlES AND LIMITATIONS.
&icrovolts. The use oiavc precludes excessive audio out-
1-32. GENERAL. The radio receiver provides optimum put when tuning through a carrier signal relatively
receiver performance when properly installed and oper- strong compared to the setting of the "AUDIO GAIN"
ated. The antenna input connector provides for connec- control, and compensates for the possible fading char-
tion to a balanced doublet or single-wire antenna sys- acteristics of the signal over long or short intervals of
tem. The 95-ohm cable used provides flexibility as to the time.
relative locations of the receiver and the antenna system.
In use, the receiver may be subjected to temperatures
within the range of O°C (32°F) to 60°C (140°F), at
relative humidities as high as 95 percent. After a 15-
minute warmup period, the frequency stability ranges
from 0.001 percent to 0.01 percent of the signal fre-
quency. The receiver incorporates advanced design and
shielding to reduce radiation characteristics to a mini-
mum. This enables the receiver to maintain performance
in multi-receiver installations and to comply with ship-
board regulations. The receiver embodies the necessary
chassis rigidity to withstand severe vibration and shock.
It is adequately treated with fungicidal varnish to con-
dition it for tropical use.
1-33. PRESELECTION. The image frequency of any
signal tuned in on the receiver is attenuated to the ex-
tent that if the desired signal strength were one micro-
volt, the strength of the image frequency signal would
have to be at least 4000 microvolts (at
7.4
mc) to de-
velop the same receiver output. At other signal fre-
quencies the strength of the image frequency signal
would have to be greater. Also, if the receiver is tuned
to 600 kc, a 455-kc signal input to the receiver will have
to be 2700 microvolts to produce the same effect as a one-
microvolt signal at 600 kc. For frequencies other than
600 kc, this 455-kc i-f rejection ratio of the receiver is
better. Likewise, the 3955-kc i-f rejection ratio of the
receiver is at least 50,000 for any signal frequency on
bands 4, 5, or 6, the frequency bands for which the 3955-
kc
i-f
channel of the receiver
is
operative.
1-34. SENSITIVITY. The receiver threshold sensitivity
(minimum signal input to develop its rated a-f output) is
1-35. SELECTIVITY. The bandwidth, or selectivity,
of the receiver is adjustable to provide for the reception
of a signal under varying conditions of propagation.
Since noise, regardless of its origin, is of the pulse type
and comprises all frequencies, the noise level in the
audio output of the receiver depends upon the overall
selectivity of the receiver. The "BROAD" settings of the
"SELECTIVITY" switch (22, figure 3-1) produce more
noise output than the "SHARP" settings. In the presence
of noise, it is advantageous to restrict the overall re-
ceiver bandwidth to the narrowest possible, consistent
with intelligible reception. However, the bandwidth rr-
quirements vary with the type of signal being received.
For phone reception, the i-f channel bandpass require-
ments are 455 kc
+
2500 cps. This is also the require-
ment for single-tone telegraph reception. For keyed c-w
operation such as slow-speed telegraph, teletype (60
words per minute), and high-speed telegraph (400 words
per minute), the requirements are 455 kc
+-
50 cps, 455
kc
i:
75 cps, and 455 kc
+
600 cps, respectively. When
two tones, one for "mark and another for "space", or
two channels are used, the bandwidth requirements are
doubled. The bandwidth requirements also depend upon
the frequency stability of the received signal and the
frequency stability of the receiver itself. For reception
of a c-w signal, a narrow bandwidth can be used. To
provide for reception of phone signals, the bandwidth
may be reduced to reduce noise output; however, the
use of a narrower bandwidth results in loss of the
higher-frequency audio components in the received sig-
nal and may impair the intelligibility of reception. To
provide for reception of telegraph or teletype signals,
