NB UNIT (PB- 1 994)
A
portion of the 8 9 8 7 . 5 kHz IF signal is fed to
pin 4 of the noise blanker unit and appears at
n oise blanker mixer Q 2 0 9
( 3 SK40M ) ,
where the
8 5 3 2 . 5 kHz signal generated by Q2 1 1
( 2SC 1 8 1 5 Y )
is mixed with t h e in coming signal to produce a
4 5 5 kHz signal. The 45 5 kHz signal is then ampli
fied by Q2 1 0
( 3 SK40M ) .
When a carrier or a noise-free modulated signal is
received, the 45 5 kHz signal (with its correspond.!
ing strength) is rectified by D2 0 2 and D2 0 3 (b oth
IN2 70)
to charge C2 3 5 . There is no discharge l o op
for C2 3 5 ; there fore , signals which exceed the
charged voltage e stablished by the reference
voltage on C2 35 will not pass through D 2 0 2 and
D2 0 3 . Accordingly, there will be no voltage drop
across
Rz 4 7 ,
and Qz 1 2
( 3 SK5 1 -0 3 )
will conduct
as the gate voltage approaches z ero p o tential .
When Q2 1 2 conducts, the d rain voltage at pin 2
of
the printed board will drop.
The drain of Q2 1 2 is directly connected to the
base of Q 3 0 3
( 2SC I 8 1 SY)
in the F I LTER UN IT.
As the drain voltage o f Q2 1 2 drops, the base
voltage of Q 3 0 3 d rops, turning off Q 3 0 3 . The
collector voltage will then increase, producing
a forward bias to D 3 0 1
( 1 S l 007 ) .
As D 3 0 1 con
ducts, the signals will pass normally through the
circuit .
When pulse-type n oise is received which exceeds
the charged re ference voltage established by C 2 3 5 ,
D2 0 2 and D2 0 3 will permit negative-going pulses
to turn Q2 1 2 off. The drain voltage will rapidly
increase as it turns o ff.
As the drain voltage increases, Q 3 0 3 wili turn on
and the collector voltage will decrease. Accord
ingly , D 3 0 1 will be biased to block the signal .
Thus, when pulse-type n oise is received , the
signal passage will b e b lanked off momen taril y.
15 ALC METER
ALC
PRO�SSOR
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NB I N
NB ON
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-- 2 6-
Summary of Contents for FT-902DM
Page 1: ...INSTRUCTION MANUAL FT 902DM YAESU MUSEN CO LTD TOKYO JAPAN ...
Page 54: ...YA E SU v 8103 A ...
Page 55: ......
