4 - 2
A 2nd LO signal (45.9 MHz) is produced at the PLL circuit
by tripling it’s reference frequency (15.3 MHz).
The 2nd IF signal from the 2nd mixer section (IC12, pin 3)
passes through the ceramic filter (Wide: FI1, Narrow: FI1,
Fl2) to remove unwanted heterodyned frequencies. It is then
amplified at the limiter amplifier section (IC12, pin 5) and
applied to the quadrature detector section (IC12, pins 10,
11) to demodulate the 2nd IF signal into AF signals.
The demodulated AF signals are output from pin 9 (IC12)
and applied to the base band IC (IC2).
4-1-5 AF AMPLIFIER CIRCUIT (MAIN UNIT)
The AF amplifier circuit amplifies the demodulated AF sig-
nals to drive a speaker. This transceiver employs the base
band IC which is composed of pre-amplifier, expander,
scrambler, MSK de-modulator, etc. at the AF amplifier sec-
tion.
The AF signals from the FM IF IC (IC12, pin 9) are passed
through the DA switch (IC42, pins 2, 15) and are then
applied to the base band IC (IC2, pin, 23). The signals are
amplified at the AF amplifier section in the base band IC (IC2,
pin 23), and are then applied to the high-pass filter and low-
pass filter section of it.
The filtered signals pass through the high-pass filter to sup-
press unwanted harmonic components. The signals pass
through (or bypass) scrambler and expander sections. The
signals are amplified at the amplifier section in the base
band IC (IC2).
The output signals from IC2 (pin 20) are applied to the AF
volume (IC8, pins 15, 16), and are then applied to the AF
power amplifiers (IC34 pins 3, 4, IC38 pins 1, 4) after pass
through the analog switches (IC42 pins 3, 4 and IC43 pins 1,
15).
The power amplified AF signals are applied to the internal
speaker (IC-F1700 series only) that is connected to J6 via
[EXT SP] jack (J5).
4-1-6 SQUELCH CIRCUITS (MAIN UNIT)
• NOISE SQUELCH
A squelch circuit cuts out AF signals when no RF signals
are received. By detecting noise components in the AF sig-
nals, the squelch circuit switches the AF amplifier controller.
Some noise components in the AF signals from the FM IF
IC (IC12, pin 9) are passed through the D/A converter (IC8,
pins 1, 2). The signals are applied to the active filter section
in the FM IF IC (IC12, pin 8). The active filter section filters
and amplifies noise components. The amplified signals are
converted into the pulse-type signals at the noise detector
section. The detected signals output from pin 13 (NOIS) via
the noise comparator section.
The “NOIS” signal from the FM IF IC is applied to the MAIN
CPU (IC23, pin 37). Then the MAIN CPU analyzes the noise
condition and outputs the AF mute signal as “AFON” from
the pin 19 to the AF power controller (Q28, Q29, D34, D36).
• CTCSS AND DTCS
The tone squelch circuit detects tone signals and opens the
squelch only when receiving a signal containing a matching
subaudible tone (CTCSS or DTCS). When tone squelch is in
use, and a signal with a mismatched or no subaudible tone
is received, the tone squelch circuit mutes the AF signals
even when noise squelch is open.
A portion of the AF signals from the FM IF IC (IC12, pin 9)
pass through the low-pass filter (IC1, pins 8, 10) to remove
AF (voice) signals, and are then applied to the amplifier (IC1,
pins 5, 7). The amplified signals are applied to the CTCSS
or DTCS decoder in the MAIN CPU (IC23, pin 46) via the
“CDEC” line. The MAIN CPU outputs the AF mute signal as
“AFON” from the pin 19 to the AF power controller (Q28,
Q29, D34, D36).
• AF AND MIC AMPLIFIER CIRCUITS
Base band IC
(IC2)
AF signal
from FM IF IC (IC12, pin 9)
23
20
AF
AMP
IC38
Speaker
IC5
IC43
IC1
IC8
D12
Microphone
AMP
3
7
4
FM/PM switch
D/A converter
FM mod.
LPF
"CTCSS/DTCS" signal from
D/A conveter IC (IC8, pins 9, 10)
"TONE" signal from
D/A converter IC (IC8, pins 21, 22)
to TX VCO circuit
(Q12, D8, D9)
3
AMP
IC34
FRONT UNIT A/B
3
5
4
IC42
15 3
4 1
4
IC8
16
3
5
3
4
1
2
15
IC43
6
7
1
IC8
IC9
AMP
2
1
15
IC42
