Icom IC-551D Instruction Manual page 32

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. OUTLINE
The 1C-551D employs a digital phase locked loop (PLL) circuit as the local oscillator for both
transmit and receive.
The output of the PLL circuit is approximately 9MHz below the receive
frequency, thereby spurious is kept to a minimum. |
(Voltage Controlled Oscillator).
The Pass Band Tuning unit acts as a low distortion RF speech processor. This enables greater talk
. power and better results in DX operation.
In the CW mode, the set uses the carrier for USB which is shifted about +500Hz. This carrier is
fed to the transmit mixer directly.
|
In the AM mode, the set uses the carrier for USB. The same 9.0115MHz SSB signal as for USB and
a part of the carrier signal is fed to the transmit mixer, and an AM (A3H, single side band with
full carrier) signal is obtained.
In the FM mode, the set uses another crystal oscillator to produce the 9.0115MHz signals which
are direct-frequency modulated. (Only when the optional FM unit is installed.)
The SSB, CW, AM or FM signal is mixed with the local oscillator output from the PLL circuit,
which is the same as that of the receiver section, and then amplified, filtered, and sent to the
antenna.
RECEIVER CIRCUITS
ANTENNA SWITCHING CIRCUIT
Signals from the antenna connector are fed to the two-stage helical cavity filter L29 and L30,
through T/R relay RL1 which are turned ON in the transmit mode, and through J3 the receiver
antenna output terminal and J4 the receiver input terminal.