Yaesu FT-901DM Instruction Manual page 28

NB UNIT
(PB- 1 703)
A p ortion o f t h e 8 9 8 7 . 5 kHz IF signal is fed to
pin 4 of the n oise blanker unit and appe ars at
n oise blanker mixer Q 2 0 9
8 5 3 2 . 5 kHz signal generated by Q2 1 1
i s mixed with the in coming signal t o produce a
4 5 5 kHz signal . The 4 5 5 kHz signal is the n ampli­
fied by Q2 1 0 ( 3 S K40M ) .
When a carrier o r a n oise-free modulated signal is
received, the 4 5 5 kHz signal (wi th its correspond�
ing strength ) i s rectified by 0 2 0 2 and 0 2 0 3 (b oth
1 N60) to charge C2 3 5 . There is no discharge loop
for C 2 35 ; there fore , signals which exceed the
charged voltage e stablished
voltage on C2 3 5 will not pass through 0 2 0 2 and
0 2 0 3 . Accordingly, there will be no voltage drop
across R 2 4 7 , and Q2 1 2
as the gate voltage approaches z ero p otential .
When Q2 1 2 conducts, the d rain vol tage at pin 2
of the prin ted board will drop .
020 1
2SKl9GR
NB I N
( 3 SK40M ) , where the
( 2 SC 1 8 1 5 Y )
by the reference
( 3 SK40M ) will conduct
The drain of Q 2
base o f Q 3 0 3 ( 2SC1 8 1 5Y )
As the drain v oltage o f Q2
voltage o f Q 3 0 3 drops, turning o ff Q 3 0 3 . The
colle ctor vol tage will then increase, producing
a forward bias to 0 3 0 1
ducts , the signals will pass normally through the
circuit.
When pulse-ty pe n oise is re ceived which e x ceeds
the charged re ference voltage establishe d by C 2 3 5 ,
0 2 0 2 and 0 2 0 3 will permit negative-going pulses
to turn Q2 1 2 off. The drain voltage will rapidly
in cre ase as it tu rn s o ff.
As the drain voltage increase s, Q 3 0 3 will turn on
and the colle ctor voltage will de crease. Accord­
in gly , 0 3 0 1 will be biase d to block the signal .
Thu s, when pulse-ty pe n oise
signal p assage will be blanked off momen taril y .
--2 6 -
2 is direc tly connected to the
1
in the F I LTER UN IT.
2 drops, the base
1
( 1 S 1 00 7 ) . As 0 3 0 1 con­
is received ,
0 20 3
3SK59GR
ALC !.ETER
the