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CHAPTER 6. FILTER, RADIO INTERFERENCE F-1 139/GRC-1 59(V)
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TM-11-5820-554-12 Radio Set AN/FRC-93(V)1 AN/FRC-93(V)2 AN/FRC-93(V)3. AN/FRC-93(V)4 AN/FRC-93(V)5 AN/FRC-93(V)6 AN/FRC-93(V)7 Manual
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Figure 6-2. Noise blanker, block diagram.


TM 11-5820-554-12
the requirements for noise blanker gain setting.
the blanking pulses are short, while the greater noise
Whenever the noise level appears to have risen, reduce
bursts develop longer blanking pulses. Transformers
the noise blanker gain, and readjust for the threshold
T1 and 12 and the gating diodes are arranged in a
condition described above. If the noise blanker fails to
balanced modulator configuration so that any noise
reduce the noise level, turn it off. The repetition of the
which results from the gating action is canceled and
noise pulses may be too rapid for the blanker to gate, or
prevented from entering the receiver circuits.  Any
a strong adjacent channel carrier may be causing erratic
discontinuity of signal resulting from the gating action
blanking.
is compensated for by tuned circuit restoration in the
following stages of the receiver. Both sections of V4
6-6. Circuit Functioning (fig. 6-2)
serve to isolate the noise-operated gate circuit from the
The first three RF amplifier stages are connected as a
receiver circuits. Vif isolation amplifier V4A provides
40-MHz tuned RF amplifier.  The gain of the RF
only enough gain to compensate for the small loss in the
amplifier is controlled by the gain control. The output
gate circuit, so that overall gain through the noise
of the third RF amplifier (V3A) is limited by the action
blanker is approximately unity. Filament power, B+
of limited diode CR8.  The output of the third RF
power, and bias voltage are taken from the Collins
amplifier is detected by noise detector CRT.  A
KWM2/2A power supply.
resistive-capacitive network determines the length of the
6-7. Limitations
blanking pulse.  The audio component of the noise
detector is limited by limited CR2 and applied to the
The noise blanker circuitry has the following three
first pulse amplifier V3B). Any negative portion of the
limitations which decrease the blanking efficiency:
output waveform is clipped by limited CR 4.  The
a. Noise pulses which have no energy distribution
positive portion is applied to the second pulse amplifier.
at 40 MHz will occur in the frequency spectrum of the
The output of this amplifier is applied to the third pulse
radio receiver range.  The noise blanker will not
amplifier. Pulse shaper CR3 is connected between the
generate a blanking pulse and will permit passage of
output and input of V1B. The output of V1B is applied
these noise pulses.
through threshold gate CR7 to the center tap of T1.
b. A very strong signal in the passband between
Variable IF input from J22 in the transceiver is applied
the first and second mixers can be modulated by the
through variable intermediate frequency (vif) cathode
blanking pulses. This modulation process will cause
follower V4B to the primary of transformer T1. The
sidebands in the passband which result in decreased
bias applied to threshold gate CR7 keeps it cut off and
blanking efficiency.  To minimize this modulation
at high impedance to the low level pulses, but high level
effect, a blanker on-off control and blanker RF gain
pulses overcome the bias and pass into the gate circuit.
control are provided on the Collins KWM-2/2A front
Gating diodes CR5 and CR6 are biased to conduction
panel.
for normal free operation.  However, when a high
c. Some corona noise and static disturbances have
amplitude noise burst occurs, the positive-going pulse
a repetition rate in excess of one hundred thousand
passes through threshold gate CR7 and cuts off both
pulses per second. The blanking efficiency decreases as
CR5 and CR6. This action effectively disconnects the
the pulse repetition rate exceeds five thousand pulses
variable IF signal for the period of the blanking pulse.
per second.
The length of the blanking pulse varies from a few
6-8. Operator Maintenance Instructions
microseconds to a maximum of 30 microseconds.
The noise blanker has no operator controls.
No
Blanking pulse length is governed by the magnitude of
operator maintenance is required.
the noise pulse appearing at the noise blanker antenna.
For short duration noise disturbances in the variable IF,
6-9. Deleted.
Change 6 6-2


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