Theory of Operation

internal pulse width modulator. Q588 is the current sense, sampling current across R690 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, 15 V, and +5 V.

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.

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1705A Spectrum Monitor

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Image 87
Tektronix 1705A instruction manual Low Voltage Power Supply Diagram