Icom IC-280E Instruction Manual page 13

SECTION Vil THEORY OF OPERATION =
'General
The 1C-280E has а digital Phase Locked Loop circuit for its local oscillator.
The PLL circuit is controlled with a programmed microcomputer developed by ICOM.
The band edges, memory channels and duplex setting are programmed in the computer.
The received signal is mixed with the output signal of the PLL circuit and converted into the
10.695MHz IF signal. This signal goes into a crystal filter to reject any nearby signals and then is
mixed with the 10.24MHz signal, which is also used as the reference frequency signal for the PLL
circuit, and converted to a 455KHz IF signal. After it is amplified, the signal is fed to the ceramic
discriminator for demodulation.
In the transmit mode, the output frequency of the PLL circuit is shifted to the transmitting
frequency and the signal is amplified up to 10 watts with the PA module.
Frequency modulation
is accomplished in the PLL oscillating circuit. 1
RECEIVING UNIT
Antenna Switching Circuit
ВЕ Circuit
IF Circuit
IF signals from the drain of ОЗ are filtered by a crystal filter and fed to the gate of the second
mixer O4.
The reference frequency signal of the PLL circuit is used for the local oscillator signal
of the second mixer. The signals are converted to 455KHz signals appear at the drain of O4 and
are amplified about 5098 by IC1 after being filtered by a high-quality ceramic filter. Then the
signals are amplified about 5 - 15dB by the variable amplification amplifier O5.
The S-meter functions with both the detected signals by D7 and D20 and by D5 and D6. By
detecting at two places, the S-meter functions with weak signals, but never scales out even with
strong signals.
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