Transmitter Circuits - Icom IC-2710H Service Manual

4-2 TRANS MITTER CIRCUITS
4-2-1 MICROPHONE AMPLIFIER CIRCUIT
The microphone amplifier circuit
from the microphone to a level needed for the modulation
circuit. The microphone amplifier circuit is commonly used
for the both VHF and UHF bands.
The AF signals from the microphone are applied to the MIC
mute switch (017 8 ) and then amplified at the 0175. Then
the amplified signals are applied to the IDC limiter amplifier
(IC25b pin 5). The output signals from the IDC limiter
amplifier (IC25b pin 7) are passed through the splatter filter
(IC25a pins 3, 1) and then applied to each VCO circuit as a
"MOD" signal via the deviation adjustment pot.
4-2-2 VHF MODULATION CIRCUIT
The modulation circuit modulates the oscillating signal (RF
signal) using the microphone audio signals.
The "MOD" signal changes the reactance of a diode (D15)
to modulate the oscillated signal at the VHF-VCO circuit
(01 8 , 019). The VCO output is buffer-amplified at 017,
016 and 015, and is then applied to the transmit/receive
switching circuit (D41, D149) via the low-pass filter (L20,
C75, C76).
4-2-3 VHF DRIVE AMPLIFIER CIRCUIT
The drive amplifier circuit amplifies the VCO oscillated
signal to the needed level at the power amplifier.
The signal from the transmit/receive switching circuit {D149)
is passed through the type attenuator circuit (R24-R26)
"re"
and then amplified at the pre-drive amplifier (011 )
amplified signal is amplified again at the drive amplifier
(01 O) to obtain approx. 26 dBm.
VHF POWER AMPLIFIER CIRCUIT
4-2-4
IC4 is a power module which provides more then 50 W of
output power with a 13. 8 V DC power source.
An RF signal from the drive amplifier (010) is applied to IC4
(pin 1 ). The amplified signal from the power amplifier (IC4
pin 4) is then applied to the antenna connector via the
transmit/receive switching circuit (D5) and low-pass filter.
VHF APC CIRCUIT
from VHFLO
­
switch (0149)
. amplifies audio signals
The
.
Drive amp.
Q10
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to UHF drive and power
amplifier circuits
4-2-5 VHF APC CIRCUIT
The APC circuit protects the power module (IC4) and drive
amplifier (010) from a mismatched output load and
stabilizes transmit output power.
The APC detector circuit (D7 and D6) detects forward
signals and rectified signals respectively.
voltage is at a minimum level when the antenna is matched
at 50 Q and increases when it is mismatched. The combined
voltage is applied to the APC amplifier (IC7) and compared
with a reference voltage which is supplied from the D/A
control IC (IC19 pin 14).
The output voltage from IC7 (pin 4) is applied to the APC
control circuit (055, 056) to control the bias voltage of the
PA module (IC9) and drive amplifier (010).
4-2-6 UHF MODULATION CIRCUIT
The audio signals from the microphone amplifier circuit
(described in Section 4-2-1) are applied to the UHF-VCO
circuit.
The audio signals change the reactance of a diode {D107)
to modulate the
(0123, D145).
buffer amplifiers (012
switch circuit {D105, D148).
4-2-7 UHF POWER AMPLIFIER CIRCUIT
IC11 is a power modulo/llJhich provides a stable 35 W (at
13.8 V DC) of output p9wer.
The pre-drive (0106, 0107), drive amplifier (0105) and
power amplifier (IC11) amplify the VCO oscillating signal to
an output level. The output signal passes through the APC
detector circuit {D91, D92) and bandpass filter, and is
applied to the antenna connector.
UHF APC CIRCUIT
4-2-8
The APC detector circuit (D92 and D91) detects the forward
and rectified signals, respectively.
voltages detected by the APC detector with the reference
voltages. When the detected voltage exceeds a reference
voltage, IC7 reduces the bias current of IC11 (pin 4) using
056 and 055 to decrease the RF output power.
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"LOW" signal from the IC19 pin 14
"TX(VTX or UTX)" signal
4 -3
The combined
IC7 compares the