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Theory of Operation
4-14
1705A Spectrum Monitor
internal pulse width modulator. Q588 is the current sense, sampling current
across R 690 to provide a voltage level to the sensing input of U583. If the level
on the over current sense input (Sen) gets too high, U583 shuts down to prevent
damage.
L695 and C697 are the output filter providing a dc output that can be switched
onto the rear-panel L--Band connector to power a Block Down Converter.
DS698 is an LED indicator light which is lighted when the +18 V supply is
operating into a normal or no-load condition. If the +18 V output is shorted, the
light will extinguish.
Low Voltage Power Supply
Diagram 7
The Low Voltage Power Supply converts the mains line voltage (90 -- 250 Vac)
to supply the power requirements of the instrument. The voltages supplied by
the Low Voltage Power Supply are +40 V, ᐔ15V,and+5V.
The Low Voltage Power Supply is called a Flyback Switcher. When switcher
mosfet Q9 is turned on, its drain voltage drops to approximately 0 V. The
current through the 350 H primary winding of T3 begins ramping up. The
voltages present at all secondaries is such that the rectifier diodes are reverse
biased. Energy is being stored in the magnetic field of T3. When Q9 turns off,
the drain voltage “flies back” in a positive direction. Current now flows in all of
the secondary windings and supplies power.