Selectivity Control; Crystal Phasing Control; Automatic Noise Limiter; A Vc-Bfo" Switch - Hallicrafters SUPER SKYRIDER SX-28-A Instruction Book

from the
.r~ceiver
once: you have become used to their
effects on the reception of various types of signals. .
(8)The SELECTIVITY control acts as a shutter or gate
and varies the width of the path on which signals reach
the second detector of the receiver. Six different selectiv-
ity steps are provided so that you can successfully cope
with different degrees of interference. Reference to Fig. 1
and Fig. 4 will show, graphically, how the control trims
the width of the signal so that what interference might
b~
present in the signal's skirts or sidebands is effectively
clIpped off. Should an interfering signal lap over into the
desired signal, adjustment of the SELECTIVITY control
will reduce that interference. '
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FIG. 1-1. F. SELECTIVITY
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KILOCYCLES OFF RESONANCE
Once more refer to Fig. 1 and Fig. 4 and recognize the
fact that with the control set in the BROAD IF position,
the signal proper and all its parts, which are combined in
the side bands, or skirts, will
be
passed to the 2nd detector,
audio amplifier, and then Speaker. As the selectivity of the
receiver is increased from BROAD·IF to XTALSHARP,
the gate, or admittance path, is so narrowed that only the
main ~rtion of the signal is allowed to pass through. This
fact and its effect on the quality of reproduction is readily
appreciated 'by listening to a signal and noting the reduction
in higher frequency response in the more. selective settings
of the switch. (See Fig. 10 and Fig. 11)
At this point, it is suggested that the CRYSTAL
SHARP setting be used only in cases of extreme inter-
ference-the receiver must then be tuned exactly to the
signal. Only then will
~hesignal
be intelligible because
you have dipped off its sidebands in which the sibilants
and overtones are embodied.
The CRYSTAL SHARP position of the selectivity
switch is to be used principally for the reception of code,
or CW, signals. By proper associated operation of the
CRYSTAL PHASING control true single signal opera-
tion and the maximum in selectivity can be obtained
(crystal circuit discussed in detail in the summary of
related circuits). See Fig. 3. .
(9) CRYSTAL PHASING CONTROL
The Phasing Control is in the circuit on three positions
of the selectivity control namely-XTAL Sharp, XTAL
Medium andXTAL Broad.
The control is used to remove heterodyne interference
as well as to minimize other forms of interference having
a predominance of high frequency components-such as
static and interference from electrically operated devices.
(lO)The A.N.L., or Automatic Noise Limiter, materially con-
I
tributes to the satisfactory operation of the receiver by
limiting objectionable interference caused by ignition
systems or other man made causes of electrical disturb-
ances. With the A;N.L. control retarded to the left as
far as it will go, or until the A.N.L. switch is heard to
operate-the noise limiter circuit is not functioning.
Turning the control to the right closes the switch which
is mounted on the control. The noise limiter is now op-
erating. Progressively turning the control clockwise
varies the threshold at which the noise limiter starts to
take hold. The setting at which the control will be left
depends entirely on the type and amount of interference
present as well as the signal strength. The noise limiter
should be judiciously adjusted because through its opera-
tion 'the desired signal can even be eliminated or badly
distorted which destroys its usefulness. Only after you
have become familiar with the operation of this control
by actual practice can you determine how far it shoukl
be advanced before the best compromise between noise
and
sig~
is obtained. (See Fig. 6.)
( 11) The A VC-BFO OFF-ON switch performs a dual func-
tion. The A'VC circuit should be operating for the recep-
tion of telepl.tone, or modulated, signals in order to
reduce fading toa minimum. As previously mentioned,
'the functioning of the S Meter is dependent upon A VC
action so the switch must be in the AVC ON position
when the, S meter is used to measure relative' carrier
intensity.
Inasmuch as the AVC circuit levels ail signals toa
predetermined value (See Fig. 7) no one signal can
overload the receiver and cause distortion. At times, in
searching for distant or weak signals, it might be
desirous to use the full sensitivity of the Model SX-28-A.
In that case place the AVCswitch in the A VC OFF posi-
tion. Remember that with the receiver operating with
no A VC action, strong signals will overload the input
circuit with resultant distortion. Under such a condition
of operation the sensitivity of the set. must be then con-
trolled, manually, by properly retarding the RF Gain
control until you have reached the point below which
overloading takes place.
The other function of this switch is to turn on the
Beat Frequency oscillator. When receiving code signals,
a beat 'note is absolutely essential. \\ ith the BFO switch
in the ON position, each signal tuned in will be accom-
panied with a' peat note or whistle. For proper adjust-
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