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4-1-8 2ND IF AND DEMODULATOR CIRCUITS
(RF AND LOGIC UNITS)
The 2nd mixer circuit converts the 1st IF signal to the 2nd IF
signal. A double conversion superheterodyne system (which
converts receive signals twice) improves the image rejection
ratio and obtain stable receiver gain.
The FM IF IC (LOGIC unit; IC701) contains 2nd local oscil-
lator, 2nd mixer, limiter amplifier, quadrature detector and S-
meter detector circuits.
The filtered 1st IF signal from the 1st IF filter (RF unit; FI601
or FI602) is mixed with the 2nd LO signal at the 2nd mixer
(LOGIC unit; IC701) to produce the 450 kHz 2nd IF signal.
The 2nd IF signal passes through or bypasses (WFM mode
signal) 2nd IF filter (FI701) where unwanted heterodyne sig-
nals are suppressed via the mode switch (LOGIC unit;
D701, D702). The filtered signals are applied to the limiter
amplifier section (LOGIC unit; IC701, pin 5), and then
applied to the quadrature detector section to demodulate the
2nd IF signal into AF signals.
The demodulated AF signals are output from pin 9 of the IF
IC (LOGIC unit; IC701) and are applied to the AF circuit.
4-1-9 AF AMPLIFIER CIRCUIT (LOGIC UNIT)
The AF amplifier circuit which is included a low-pass and
high-pass filter, AF mute switch, AF volume controller and
AF amplifier amplifies the demodulated AF signals to drive a
speaker.
The demodulated AF signals (DETO signal) from the FM IF
IC (IC701) are passed through the AF filter (low-pass and
high-pass filters). The filtered signals are applied to the AF
mute switch (Q361) which is controlled by "RM/MM" signals
from the CPU (IC1, pin 31), and are then applied to the elec-
tric volume control circuit (IC202, IC203). The level con-
trolled AF signals are output from volume IC (IC202, pin 7)
and are then applied to the AF amplifier (IC201, pin 4). The
AF signals are then applied to the internal speaker (SP1) via
the [EXT SP] jack (LOGIC unit; J3) when no plug is con-
nected to the jack.
The AF filter circuit (IC241) removes AF signals below 300
Hz (CTCSS signals) for clear AF output and these are
applied to the CPU (IC1, pin4) for CTCSS squelch detection
via the "CTCIN" line.
4-1-10 SQUELCH CIRCUIT (LOGIC UNIT)
• NOISE SQUELCH
The noise squelch circuit cuts out AF signals when no RF
signals are received. By detecting noise components in the
AF signals, the squelch circuit switches the AF mute switch.
A portion of the AF signals from the FM IF IC (IC701, pin 9)
are applied to the active filter section (IC701, pin 7, 8). The
active filter section amplifies and filters noise components.
The filtered signals are applied to the noise detector section
and output from IC701 (pin 13) as "NOISE" signal.
The "NOISE" signal from IC701 (pin 13) is applied to the
CPU (IC1, pin 12). The CPU analyzes the noise condition
and outputs the "RM/MM" signal to AF mute switch (Q361).
Even when the squelch is closed, the AF mute switch
(Q361) opens at the moment of emitting beep tones.
• TONE SQUELCH
The tone squelch circuit detects AF signals and opens the
squelch only when receiving a signal containing a matching
subaudible tone (CTCSS). 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 (IC701, pin9)
passes through the AF filter (IC241) to remove AF (voice)
signals and is applied to the CTCSS decoder inside the CPU
(IC1, pin 4) via the "CTCIN" line to control the AF mute
switch.
4-2 TRANSMITTER CIRCUITS
4-2-1 MICROPHON AMPLIFIER CIRCUIIT
(LOGIC UNIT)
The microphone amplifier circuit amplifies the audio signals
from the microphone, within +6 dB/octave pre-emphasis
characteristics (300 Hz–3 kHz), to a level needed for the
modulation circuit.
The AF signals from the internal microphone (MC1) or exter-
nal [MIC] jack (J4) are applied to the microphone (limiter)
amplifier (IC301, pin 3) which has +6 dB/octave pre-empha-
sis characteristics, and are then passed through the low-
pass filter (IC301, pin 6 and 7). The filetered signals are
applied to the modulation circuit for each band in the RF unit
via the band switch (Q304: for 144 MHz band, Q305: for
UHF band, Q306: for 50 MHz band, Q309: for 120MHz
band) as the "MOD" signal.
4-2-2 MODULATION CIRCUIT (VCO AND RF UNIT)
The modulation circuit modulates the VCO oscillating signal
(RF signal) using the microphone AF signals.
(1) 50 MHz band
The signals from the limiter amplifier (RF unit; IC301)
changes the reactance of a diode (RF unit; D341) to modu-
late the oscillated signal at the 50-VCO circuit (RF unit;
Q341, D341, L341–343). The modulated signals are ampli-
fied at the buffer-amplifier (Q342) and the LO amplifier
(Q821). The amplified signals are applied to the drive/power
amplifier circuits for VHF band.
(2) 144 MHz band
The signals from the limiter amplifier (RF unit; IC301)
changes the reactance of a diode (VCO unit; D302) to mod-
ulate the oscillated signal at the 144-VCO circuit (VCO unit;
Q311, Q312, D302, D311–D313). The modulated signals
are amplified at the buffer-amplifier (Q312, Q313) and the
LO amplifier (Q822). The amplified signals are applied to the
drive/power amplifier circuits for VHF band.
4 - 3
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