Icom IC-551D Instruction Manual page 37

but this is completely removed Бу the crystal fitter РІ1 on the MAIN unit, and the final signal is а
clean and powerful SSB signal.
TRANSMIT MIXER CIRCUIT
High quality ҒЕТ% 06 and O7, and coils 1-17 and L18 work as а double-balanced mixer to provide
low spurious generation and excellent mixer characteristics. The local oscillator signals are supplied
to L18 through D7 from the PLL circuit and mixed with the AM, SSB or CW signals fed through
D8 from ЕП. Both added and subtracted frequencies are generated Бу this mixer, but only the
signals of the added frequency are filtered through the transmit band-pass filter L15 and L16.
BUFFER AMPLIFIER CIRCUIT
The filtered 50MHz signals, supplied to the first gate of the dual-gate FET ОБ, are amplified to _
approximately 10mW PEP. The second gate is controlled by the ALC in the SSB mode and by the
APC in the FM mode. The output of this buffer amplifier is sent through the transmit band-pass
filter L13 and L14 to the other buffer amplifier Q4, whose output level is approximately 200mW
PEP. L11 and C35 work as a trap for the local oscillator signals.
DRIVER CIRCUIT
The output signals of O4 are amplified by ОЗ up to approximately 1.5W PEP. D5, which is con-
nected to the base circuit of ОЗ, is for temperature compensation to stabilize the bias voltage. The
idling current is controlled by R15 to eliminate cross-over distortion. |
The signals from Q3 are amplified by Q2 to obtain 10W PEP. The heat from O2 is transmitted
to the diecast chassis which radiates the heat very efficiently. D4 is for the bias voltage stabili-
zation, and the idling current is controlled by R9. The output signals of Q2 are tuned by C10 and
fed to the PA unit through J9 and P1 of the PA unit.
POWER AMPLIFIER CIRCUIT
The 10 Watt signal from the exciter stage is amplified class B push- pull i in the final amplifier stage
transistors, Q1 and O2 of the РА unit to a level of about 80 Watts. Bias voltage is treated by D1,
D2 and O3. The junction voltage of D1 and D3 is amplified by Q3 and applied to the bases of Q1
and O2, from the emitter of O3, for the bias voltage.
The thermo-switch is set at near the PA transistors, and the thermo-swiich і is turned ON when the
temperature of the PA transistors is over 80°С and the buzzer beeps for warning to stop trans-
ти а. -
The PA unit has the cooling fan to increase the efficiency of the heatsink. The cooling fan is
selectable to turn in the transmit mode only or to turn in either the transmit mode or the receive
mode.
The 80 Watt signal is fed to the filter circuit.
LOW-PASS FILTER CIRCUIT
The PA output is fed to two section low-pass filters which serve to attenuate harmonics by more
than 60dB in order to get а pure transmit output. This signal is fed to the ANTENNA connector -
through contacts of the T/R relay which is made in the transmit mode and SWR pick up coil L3,
J3 and P6.
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