Receiver Rf Circuit; Nb Circuit; Tx/Rx Section: Nb To Af - Furuno FS-1503 Service Manual

F + 1.5 kHz 30M L.P.F
Reference) Noise blanker (NB)
NB rejects pulse noise included in the receiving signal. U25 in NB detects pulse noise
and outputs negative pulses to the output terminal (BLANK). The pulse signal turns off
switching diodes CR11 and CR12 and blocks the receiving signal during the time the
pulse noise occurs.
Changing the value of a resistor R35 can adjust the noise detection level of the NB.
In receiving TLX, the NB is not operated, because it cuts top portions of the signal
pulses.
Note) The transmitting signal of 455 kHz also passes CR11 and CR12, which are ON
during transmitting.

TX/RX Section: NB to AF

The output signal from the NB passes through 455-kHz filters with bandwidth corre-
sponding to the class of emission, which reject noise out of pass band. The filters are
commonly used for both transmitting and receiving.
Note) The passing filter is changed according to the class of emission.
FIL 2 (fB = 2.4 kHz)
FIL 3 (fB = 0.4 kHz)
FIL 4 (fB = 6 kHz)
60 M H.P.F
Q1,/2
30 M L.P.F FL1
CR5
54.455 MHz
•i 2SK937•j
TP1
1Lo (F + 54.455 MHz + offset: 1.5 kHz)
Figure 2-15 Receiver RF Circuit
T11
C54
0 V
U25
BLANK
RF
DET
0 V
LEVEL
R35
Figure 2-16 NB Circuit
J3E
J2B (TLX) ---- option
H3E receiving
CR10
CR11
U1
-12
TP3
(MC1350D)
2Lo (54 MHz)
NB
CR11
T12
CR12
C57 R34
Vcc
Q10
CHECK
NB OFF
HOLD
TX
2-22
FL2
U2
2.4 k
FL3
(MC1350D)
0.4 k
FL4
6.0 k
U25
455 kHz
455 kHz