Blanking Circuit for Oscilloscope

This circuit blanks the CRT starting just before retrace begins. Since the CRT sits at a -1200 volt potential, the blanking circuit must also be based at -1200 volts. Note the capacitors, C40, C41, and C43 which allow the gates that drive the intensity grid of the CRT to be isolated from the other electronics within the oscilloscope.

The described circuit functions as a blanking mechanism for a cathode-ray tube (CRT) display, specifically designed to activate just prior to the retrace period of the CRT's operation. This timing is critical as it prevents the display from showing unwanted artifacts during the retrace interval, thus ensuring a clear visual output.

The circuit operates at a high negative voltage of -1200 volts, which is the potential level at which the CRT operates. To accommodate this high voltage, the blanking circuit is designed to function within the same voltage domain. The isolation of the blanking circuit from the rest of the oscilloscope's electronics is achieved through the use of capacitors C40, C41, and C43. These capacitors serve a dual purpose: they provide the necessary coupling to allow the blanking signal to control the intensity grid while protecting the sensitive components of the oscilloscope from high voltage transients that may occur during operation.

In the schematic, the gates controlling the intensity grid are connected to the output of the blanking circuit, ensuring that the intensity of the CRT beam is modulated appropriately during the retrace period. The capacitors' values and types should be selected based on their voltage ratings and frequency response characteristics to ensure reliable performance at the operating voltage of -1200 volts.

Overall, the design of this blanking circuit is essential for maintaining the integrity of the displayed waveform on the CRT, allowing for accurate and clear visual representation of the electronic signals being measured. Proper implementation will enhance the functionality of the oscilloscope while ensuring user safety and equipment longevity.This circuit blanks the CRT starting just before retrace begins. Since the CRT sits at a -1200 volt potential, the blanking circuit must also be based at -1200 volts. Note the capacitors, C40, C41, and C43 which allow the gates that drive the intensity grid of the CRT to be isolated from the other electronics within the oscilloscope.

🔗 External reference