Receiver Circuit; Tx/Rx Section: Fo To Nb (J3E); Block Diagram Of Fil (Rx) - Furuno FS-1503 Service Manual

2.5 Receiver Circuit

PA/FIL section
The RF signal received by the antenna inputs to PA/FIL board, passes through the com-
mon L.P.F.'s for transmitting/receiving and BC-band-rejection filter, and outputs to
TX/RX board.
The BC-band-rejection filter is a 1.6 MHz H.P.F and switched ON/OFF with the jumper
wire connections at J8 and J9. The factory default connection is OFF: 2. When a BC-
band interference occurs in the receiving band higher than 1.6 MHz, the jumper wire is
set to ON:1.
ANT
E1 350 V

TX/RX Section: Fo to NB (J3E)

The receiving signal (F + 1.5 kHz) from PA/FIL board is passed through a 30 MHz L.P.F.
and amplified (G-G grounding amplification) by Q1/2. The gain is about 10 dB.
This signal is passed through 30 MHz L.P.F., mixed with a local oscillator signal 1Lo (F
+ 54.455 MHz + 1.5 kHz) by DBM CR5 (SBL-1C), converted to a signal with frequency
(F + 54.455 MHz + 1.5 kHz) ± (F + 1.5 kHz), and derives the first intermediate frequen-
cy signal of 54.455 MHz with FL1.
The first IF signal is further amplified by U1, mixed with 2Lo (54 MHz) by a ring
modulator CR10 (SMS3926-023), and derives the second intermediate frequency signal
of 455 kHz.
The second IF signal is passed through a noise blanking circuit NB (CR11/12) and FL2
(455 kHz), and amplified by U2.
The gain of U1 and U2 (MC1350D) is controlled by AGC (forward AGC).
Note) The DBM impedance is matched to 50 Ω over the wide frequency range by incor-
porating the 30 MHz L.P.P and 60 MHz H.P.F. to the input of DBM CR5, reducing spu-
rious signals generated in the DBM.
PA
T5
B1
B2
B3
B4
B5
B6
Figure 2-14 Block Diagram of FIL (RX)
J8
2
1
BK 1.6 MHz H.P.F
J8/9
1: 1.6 to 30.0 MHz
2: 0.1 to 30.0 MHz
B1: 0.1 to 2.4 MHz
B2: 2.4 to 3.6 MHz
B3: 3.6 to 6.0 MHz
B4: 6.0 to 10.0 MHz
B5: 10.0 to 18.0 MHz
B6: 18.0 to 30.0 MHz
2-21
J9
To TX/RX PCB
2
1