T Ran S Mllter Circuits - Icom IC-2100H Service Manual

a.1-7 SQUELCH CIRCUIT (MAIN and LOGIC units)
•
NOISE SQUELCH
The noise squelch circuit cuts out A F signals when no R F sig­
nals are received. By detecting noise components in the AF
sig nals, the squelch circuit switches the A F mute switch.
A portion of the A F signals from the F M IF IC (IC61 pin 9 ) are
applied to the active filter section (IC6, pin 8). The active fil­
ter section amplifies and filters noise components. The fil ­
tered signals are applied to the noise detector section and
output from pin 14 as the "SOL• signal.
The "SOL� signal from IC6 (pin14) is applied to the CPU
(LOGIC unit; IC1, pin 98). The C PU analyzes the noise con·
ditlon and outputs the "R MVT" and "A MUT" signals via the
1/0 expander I C (LOGIC unit; IC10 ) to tog gle the detector
(02 5) and AF (028) mute switches.
Even when the squelch is closed, the A F mute switch (02 8)
opens at the moment of emitting beep tones.
•TONE SQUELCH
The tone squelch circuit detects A F signals and opens the
squelch only when receiving a signal containing a matching
subaudlble tone (CTCSS). When tone squelch is in use, and
a signal with a mismatched or no subaudible tone I s
received, the tone squelch circuit mutes the A F signals even
when noise squelch is open.
A portion of the AF signals from the F M I F IC (IC6, pin 9 )
passes through the low-pass filter (LOGIC unit; IC6) to
remove AF (voice) sig nals and I s applied to the CTCSS
decoder inside the CPU (LOGIC unit; IC1, pin 1) via the
"TON EIN" line to control the
3-2 TRANSMllTER CIRCUIT
3-2·1 MICROPHONE AMPLIFIED (LOGIC unit)
The microphone amplifier circuit amplifies audio signals with
dB/octave pre-emphasis characteristics from the micro­
+6
phone to a level needed for the modulat i o n circuit.
The A F signals from the microphone are adjusted for imped­
ance - matching at the MIC sensit i v ity control circuit (IC4, 04).
The adjusted signals pass throug h the MIC mute sw i t ch ( 0 4),
and are then amplif i e d at the microphone amplifier (05) and
the limiter amplifier (IC5a) Which has a neg ative feedback
circuit for +6 dB/octave pre·emphasls.
The amplified signals are applied to the l o w-pass filter (IC5b)
to filter out RF components and are then applied to the MAIN
unit as the "MO D" signal.
3-2-2 MODULATION CIRCUIT (MAIN unit)
The modulation circuit modulates the VCO oscillating signal
( R F signal) using the microphone audio sig nals.
The audio signals ( MO D) chang e the reactance of 01 to
modulate the oscillated signal at the TX-VCO circuit (01,
02). The modulated signal is amplif i e d at the buffer amplifier
(04) and LO amplifier (05) , then applied to the drive ampli­
fiers.
and AF mute switches.
OET
a.2-3
DRIVE AMPUAER CIRCUIT (MAIN
The drive amplifier circuit amplifies the VCO oscillating signal
to the level needed at the power amplif i e r.
The AF signal from the LO amplifier (05) passes throug h the
T/R switch (05) and is amplified at the pre-drive (013) and
drive (014) amplifiers. The amplified sig nal is applied to the
power amplifier circuit.
3-2-4 POWER AMPLIFIER CIRCUIT (MAIN unit)
The power amplifier circuit amplifies the dr iver signal to an
output power level.
The RF signal from the drive amplifier (014) is applied to the
power module (IC4) to obtain 55 W (25 W for Taiwan version,
10 W for the IC- 2100-T Thailand version) of R F power.
The amplified signals is passed throug h the antenna switch­
ing circuit (07), A PC detector circuit (L18,
pass filter (L 19, L20, C62-C64) and is then applied to the
antenna connector.
Collector voltages for the driver (013) and control voltage for
the power amplifier (IC4, pin 2 ) are controlled by the A PC cir­
cuit to protect the power module from a mismatched condi·
tion as well as to stabilize the output power.
3-2..S APC CIRCUIT (MAIN
The A PC circuit protects the power amplifier from a mis­
matched output load and stabiliz es the output power.
The APC detector circuit (L10 , 08, 09)
nals and reflection signals at 08 and 09 respectively. The
combined voltage is at minimum level when the antenna
impedance is matched at 50
mismatched.
The detected voltage I s applied to the differential amplifier
(IC3, pin 3), and the power setting voltage is applied to the
other input (pin 1) for th e reference.
When antenna I m pedance I s mismatched, the detected volt­
age exceeds the power setting voltag e. The output voltag e of
the differential amplifier (IC3, pin 4) controls the input current
of the power module (IC4) and drive amplif i e r (014) to
reduce the output power via the A PC controller (018, 019).
• APC
circuit
TSV
3 - 2
unit)
08,
09), and low­
unit)
forward sig­
detects
and is increased when it is
0
to antenna
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08
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