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5.6 NB
(Noise Blanker)
BOARD
(xX54-1040-10)
(See Figure 20)
The noise blanker is designed to remove pulse noises from
received signals. The circuit switches closed during the noise
impulses
and
switches open when
noise impulses are not
present. The gate opens and closes fast enough so that the
ear hears an uninterrupted audio signal.
The receiver's second IF signal, from the MIX board, passes
through the input filter of the NB board and is amplified by
transistor Q7. The signal then feeds to the XF board.
When the NB switch is turned on, the signal (amplified by
Q8 and Q9) is separated into signal and noise components.
The noise component
(shaped into a suitable waveform by
Q4, O5 and O6) triggers the blanking gate diodes, D3 and
D4. When the diodes are triggered, the signal path is closed.
In this manner
most of the impulse noise is blocked out,
leading to higher receiver intelligibility.
FIGURE 20.
NB Board
Transistor Q2 is an AGC amplifier for the IC circuit.
625 33 25V
| NB
Xr
output
"O7
9
a
iF $
|
|
|
|
|
|
|
|
|
|
|
|
|
|
Noise Blanker
SW
NBS
rab a
ON
ANY
R31 470K
I
eet
oo
sno
—
SS
PS
a
Eat
L 2
a}
o3i .47 25V
=
@
Qi: 28c374(8)
Diz: INGO
Q2,3 : 28045818)
Da, 1S 734
Qa-6: 28A495(Y)
Ds
: 1SI555
Q7
: 38K35(GR)
Qs,9 + TA-7045M(R)
NB Unit (X54-1040-10)
FIGURE 21.
NB Schematic
26
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