Furuno FS-1503, Service Manual

Page: 24 / 136

2-17
2.4 TX/RX circuit
TX/RX section (J3E)
The AF input signal from a dynamic microphone (600
Ω
, rated input -46 dBm) is ampli-
fied by a compressor amplifier U14 (M51304L) to increase the average transmitting
power and to suppress an over-input as well.
At the next stage U5 2/2, the AF signal gain is adjusted in accordance with the radio
wave format J3E and H3E.
Further, after passing through R156 [TX GAIN], the signal is mixed with 3Lo (456.5
kHz) by a ring modulator CR22 (SMS3926-023) and converted to a signal of 456.5 kHz
±
AF (1.5 kHz).
This signal passes through a crystal filter FL2 (2.4 kHz) and derives the first IF signal of
455 kHz.
Note) The passing filter is changed according to the class of emission.
FIL 2 (fB = 2.4 kHz) J3E, H3E
FIL 3 (fB = 0.4 kHz) J2B (TLX) ---- option
FIL 4 (fB = 6 kHz) H3E receiving
With H3E, 3Lo signal (456.5 kHz) is added to the output signal from FL2 (2.4 kHz)
through R101 [H3E CARR
].
LIM
CR27
-30
FL2
2.4 k
FL3
FL4
6.0 k
0.4 k
MIC
LINE TONE
MIC ON
MIC&TONE
TP6
U5 2/2
R156
INTERCOM
-6 dB (H3E)
3Lo (456.5 kHz)
CR22
Q16 U14
COMP
TX
TP5
R101
H3E CARR
TX GAIN
(600
Ω
– 46 dBm)
FIL N/M/
W
455 kHz
Figure 2-9 AF Amplifier thru 455 kHz Circuit
The signal of 455 kHz is mixed with 2Lo (54 MHz) by a ring modulator CR10
(SMS3926-023) and converted to a signal of 54 MHz
±
455 kHz. This signal is amplified
by Q7, passed through a filter FL1 (54.455 MHz), and derives the second IF signal.
This signal of 54.455 MHz is mixed with a local oscillator signal 1Lo (F + 54.455 MHz +
1.5 kHz) by DBM CR5 (SBL-1C) and converted to a signal with frequency (F + 54.455
MHz + 1.5 kHz)
±
54.455 MHz. This signal further passes through 30 MHz L.P.F. and
derives a transmitting signal with a frequency of F + 1.5 kHz.
Note)
The output impedance is matched to 50
Ω
over the wide frequency range by in-
corporating the 30 MHz L.P.P and 60 MHz H.P.F. to the output of DBM CR5, reducing
spurious signals generated in the DBM.