Icom IC-280E Instruction Manual page 14

13
AF Circuit
The demodulated signals are fed to the de-emphasis circuit to equalize the high pitch tone which is
pre-emphasized at transmit. Then the signals are fed to the 1С4-1 attenuator element which turns
ON/OFF the audio signals by the signals from the SOL circuit. The output signal from 1С4-1 is
fed to the control unit through J1.
SOL Circuit
The signals from the demodulator circuit are also supplied to the squelch circuit. The signals are
filtered out to get approximately 20KHz noise by the parallel resonant circuit of L11 and C45.
The noise level is controlled in the attenuator element by the SOL control voltage varied by the
SOL control. The output of 1С4-2 is amplified by one of the op. amps of IC3, which has a con-
stant amplification regulated by R70. The amplified noise signal is fed to the op. amp IC3-2
comparator and is detected. This detected signal is filtered by R50 and C39 and switches 06.
O6's collector is connected to IC4-1 attenuator element and switches the audio signals.
TRANSMITTING UNIT
PA Circuit
FM signals generated in the PLL circuit have approximately 200mW output and are supplied to the
IC1 PA module on the chassis to be amplified up to 10W. This PA module was developed
especially to provide very high reliability. The output signals from the PA module are fed to a
3-stage low-pass filter to eliminate unwanted harmonics completely.
APC Circuit
RF output power is kept constant at 10 watts (or 1 watt) by this circuit, even when there is a
variation of the power supply voltage or the antenna load.
The voltage across R47 in the main unit is amplified by 1С7, of which the output controls the
power voltage of the driver in the PA module by Q1 on the chassis.
In the High power mode, 1C5-1 operates; in the Low power mode 1С5-2 operates. R52 adjusts the
detecting level in the High power mode and R64 adjusts the detecting level in the Low power mode
This APC circuit maintains a constant RF output with +15% fluctuation of the power supply
voltage and protects the PA module from any damage caused by improper load to the antenna.
For the RF output meter reading, D4 rectifies the RF signals picked up by C61 from the PA
module output. R34 adjusts the meter sensitivity.
OTHERS
Transmit Indicator Circuit
9V is supplied to the anode of the LED in the control unit through a resistor, and its cathode is
connected to the PTT switch circuit. The LED ilfuminates in the transmit mode.
Receive Indicator Circuit
The output of Squelch control O6, located on the main unit, is fed through R49, through J1
pin 4 to J2 pin 4 of the control unit. From J2 it is fed through R37 to the switching transistor
011. O11 turns the LED 09 ON and OFF.