Icom IC-720A Instruction Manual page 28

to а second DBM then то the product detector, then to Ше
Main Unit AF Amplifier. In the case of AM, the signal goes
to the AM Detector Circuit, then to the Main Unit AF Amp.
In SSB Transmit (USB or LSB) the audio signal and BFO
signal go to the DBM where a double sideband signal is
XF(SSB) AMP овм ХЕ
ЕВОМ
IF UNIT
6-5 MAIN UNIT
Composed of the AF Amplifier, MIC Amplifier, VOX
Circuit, Anti-VOX Circuit, CW Break-in Circuit, CW Moni-
tor, ALC, APC, Regulator Circuit, and the Meter Control
Circuit.
In receive, the detected AF signal from the IF unit, is
applied to a high performance and low distortion AF power
amplifier IC, and amplified and fed to the speaker.
in VOX operation, a part of the AF output signal is applied
to the ANTI VOX circuit, and its output voltage is fed to
the VOX circuit to prevent misfunction of the VOX by
audio from the speaker.
In the CW mode, the side-tone oscillator is keyed according
to your keying, and its output signal is fed to the AF power
amplifier.
A voice signal from the microphone is applied to the MIC
amplifier, amplified and fed to the modulation circuit on
the IF unit.
The ІС-720А has a protection circuit which detects reflected
power, collector current of the final transistors, and ALC
condition.
И some problem occurs, such as mismatching of the antenna,
the circuit reduces the driving power to the final power
transistors, and protects them.
6-6 DRIVER UNIT
The transmit signal from the RF Unit is amplified to 6
Watts in the Driver Unit, which is composed of three stages
of wideband amplifiers.
ЇР UNIT BLOCK DIAGRAM (Arrow shows the signal path in Receive)
(9.0115) (10.75)
X 19.7615MHz (SSB, CW, RTTY}
produced, then to the SSB Filter where USB or LSB is
selected.
During CW or AM transmit the BFO signal is sent directly to
the RF Unit, thus the BFO signal is used as a carrier signal.
TO MAIN UNIT
9.0100MHz
9.0130MHz
9.0098MHz
9.0084 75MHz
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О
5
6-7 РА ЏМТ
This unit is made up of а class AB push-pull wideband агп-
plifier, a negative feedback circuit for giving a gain flatness
response across the bands, and a thermal switch that
monitors the temperature of the final transistors and con-
trols the speed of the cooling fan. The signal from the
Driver unit is applied this unit and amplified to 100 watts.
6-8 LOGIC UNIT
This unit is composed of an incorporated CPU for treating
data such as up/down signal detection control, the dial lock
control circuit, the speed tuning circuit, the rotary relay
control circuit, code converter circuit, and the 1/О control
circuit.
The dial clock signals are generated by the photo-chopper
directly connected to the tuning knob. The up/down cont-
rol circuit detects the rotating direction of the tuning knob,
and puts out the up/down control signal for the CPU.
The CPU is a 4-bit microcomputer which has a program to
control the set. The CPU's function are determined by the
input signals, such as the dial clock signal, up/down signal
and the signals from the function switches, and the CPU
puts out signals to control the operating band, frequency,
mode, tuning steps, display, and so on.
6-9 PLL UNIT
This unit is composed of three Phase-Locked Loops for the
15% Local Oscillator, a crystal oscillator and multiplier for
the 2nd Local Oscillator, and voltage regulators for this unit.
The first loop puts out 13.15 ~ 23.14MHz with 10KHz
steps.
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