Furuno FS-1503 - NB Circuit; Receiver RF Circuit; TX;RX Section: NB to AF

2-22
1IF AGC
BLANK
TP3
CR5
54.455 MHz
2Lo (54 MHz)
CR11
-12
NB
F + 1.5 kHz
455 kHz
FL1
Q1,/2
1Lo (F + 54.455 MHz + offset: 1.5 kHz)
TP1
U2
30M L.P.F
U1
30 M L.P.F
60 M H.P.F
(
MC1350D
)
U25
FIL N/M/W
FL2
2.4 k
FL3
FL4
0.4 k
6.0 k
2IF AGC
CR10
(MC1350D)
R40
R41
•i
2SK937
•j
455 kHz
Figure 2-15 Receiver RF Circuit
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.
BLANK
RF
0 V
HOLD
CHECK
Vcc
LEVEL
R35
R34C57
CR11
CR12
T12T11
C54
DET
NB OFF
TX
Q10
0 V
U25
Figure 2-16 NB Circuit
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) J3E
FIL 3 (fB = 0.4 kHz) J2B (TLX) ---- option
FIL 4 (fB = 6 kHz) H3E receiving