3 - 1
SECTION 3
CIRCUIT DESCRIPTION
3-1 RECEIVER CIRCUITS
3-1-1 RF FILTER CIRCUIT (MAIN UNIT)
Received signals from the antenna connector pass through
the transmit/receive switching relay (FILTER board RL4317)
and are then applied to the MAIN unit via J2.
The signals pass through the protection relay (RL2),
1.6 MHz cut off high-pass filter (L2–L4, C4–C8, C629) and
are then applied to one of nine bandpass filters (including
one low-pass filter for below 2.0 MHz). These filters are
selected by the filter control signals (B0–B8) as described
in the table below.
The filtered signals pass through the 30 MHz cut-off low-
pass filter (L71, L72, C130–C134, C618), and are then
applied to the 1st mixer circuit (Q6, Q7).
• RF FILTERS USED
3-1-2 1ST MIXER AND IF CIRCUITS (MAIN UNIT)
The 1st mixer circuit converts the received signals into a
fixed frequency, 69.0115 MHz 1st IF signal using the PLL
output frequency. By changing the PLL frequency, only the
desired frequency is picked up at the pair of crystal filters
(FI1a, FI1b) at the next stage.
The IF amplifier (Q8) and resonator circuits are designed
between the filter pair. The PLL output signal (1LO) enters
the MAIN unit via J3 and is amplified at the 1st LO ampli-
fier (Q5) and then applied to the 1st mixer (Q6, Q7)
3-1-3 2ND MIXER AND IF CIRCUITS (MAIN UNIT)
The 1st IF signal from the crystal filter (FI1b) is converted
again into a 9.0115 MHz 2nd IF signal at the 2nd mixer
circuit (D52, L66, L67). The 60 MHz 2nd local signal (2LO)
from the PLL unit enters the MAIN unit via J4 to be applied
to the 2nd mixer.
The 2nd IF signal is passed through the noise blanker gate
(D15, D16) and amplified at the 2nd IF amplifier (Q16) and
then applied to one of the 9 MHz IF filters as described
below. The passed signal is amplified at the two stage 2nd
IF amplifiers (Q32, Q33) and is applied to a demodulator
circuit (D39 for H3E or IC10 for J3E and others).
• 2ND IF FILTERS USED
3-1-4 NOISE BLANKER CIRCUIT (MAIN UNIT)
The noise blanker circuit cuts off the IF circuit line at the
moment of receiving a pulse-type noise.
A portion of the 2nd IF signal between resonator circuits
(L83, L84 after stage of the 2nd mixer, D52) is amplified
at the noise amplifiers (Q9, IC8, Q11). The signal is then
detected at the noise detector (D17) to convert the noise
components to DC voltages.
The signals are then applied to the noise blanker switch
(Q13, Q14). At the moment the detected voltage exceeds
the Q13’s threshold level, Q14 outputs a blanking signal
to close the noise blanker gate (D15, D16) by applying
reverse-biased voltage. Q15 turns the noise blanker circuit
ON and OFF.
LPF or
BPF
0.5–29.999 MHz
1st mixer
Q6, Q7
1st LO:
69.5115–99.0115 MHz
Fl1a/Fl1b
Crystal
filter
Crystal
filter
69.0115 MHz
2nd mixer
D52
2nd LO: 60.0 MHz
Fl2 or
Fl3/Fl4
9.0115 MHz
Other modes
H3E
Detector
D39
Demodulator
IC10
Audio output
BFO
J3E, J2B, R3E, FSK:
FSK narrow, J2B narrow:
A1A:
9.013 MHz
9.0123 MHz
9.0116 MHz
Frequency
(MHz)
0.5–1.999
2–2.999
3–4.999
5–6.999
7–9.999
Frequency
(MHz)
10–13.999
14–17.999
18–23.999
24–29.999
Control
signal
B5
B6
B7
B8
Control
signal
B0
B1
B2
B3
B4
Entrance
coil
L49
L8
L13
L18
L23
Entrance
coil
L28
L33
L38
L43
MODE
J3E, R3E, FSK
H3E
FSK narrow,
A1A narrow
Used filter
FI2
FI3/FI4
Optional narrow
filter*
Control signal
SEL8: low, H3E8: low
SEL8: low, H3E8: high
SEL8: high, H3E8: low
• RECEIVE FREQUENCY CONSTRUCTION
*Built-in to the GMDSS versions
