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Distributed deflection plates: Difference between revisions
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Distributed deflection plates (view source)
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In [[CRT]]s, a trade/off exists between writing rate, deflection sensitivity and spot size. For example, a slower beam improves the sensitivity but hurts writing rate and spot size due to mutual repulsion between electrons. Within a given technology (e.g. mono acceleration, post deflection acceleration or microchannel plate (MCP)), these three characteristics can be traded off against each other. Improve one and one or both of the others suffer. Improve the technology and all three can be improved simultaneously. Writing rate is important for observing single short-lived events, but is not important for repetitive signals. Spot size is important in showing detail in the waveform. Sensitivity is important mostly to permit greater bandwidth in vertical amplifiers. | In [[CRT]]s, a trade/off exists between writing rate, deflection sensitivity and spot size. For example, a slower beam improves the sensitivity but hurts writing rate and spot size due to mutual repulsion between electrons. Within a given technology (e.g. mono acceleration, post deflection acceleration or microchannel plate (MCP)), these three characteristics can be traded off against each other. Improve one and one or both of the others suffer. Improve the technology and all three can be improved simultaneously. Writing rate is important for observing single short-lived events, but is not important for repetitive signals. Spot size is important in showing detail in the waveform. Sensitivity is important mostly to permit greater bandwidth in vertical amplifiers. | ||
Post deflection acceleration was probably the first technology change. It dates from the early 1950s and was a major factor in the success of the 530/540 series of oscilloscopes. Post deflection acceleration is exactly what its name implies. After the electron beam travels through both vertical and horizontal deflection plates, the beam is accelerated by a resistive helix painted on the inside of the tube from near the deflection plates almost to the phosphor screen. The end near the deflection plates is held at the potential of the plates. This assures that the electrons do not see the high voltage at the phosphor screen and can travel between the plates relatively slowly. Their slow speed allows them to be deflected easily. After passing | Post deflection acceleration was probably the first technology change. It dates from the early 1950s and was a major factor in the success of the 530/540 series of oscilloscopes. Post deflection acceleration is exactly what its name implies. After the electron beam travels through both vertical and horizontal deflection plates, the beam is accelerated by a resistive helix painted on the inside of the tube from near the deflection plates almost to the phosphor screen. The end near the deflection plates is held at the potential of the plates. This assures that the electrons do not see the high voltage at the phosphor screen and can travel between the plates relatively slowly. Their slow speed allows them to be deflected easily. After passing though the plates, the resistive helix presents an increasing voltage that gives the electrons a much higher speed, resulting in a bright spot. Since the electrons move quickly, they disperse less due to their mutual repulsion, resulting in a small spot size. This was the standard until the early 1960s. | ||
Shaped metal meshes placed after the deflection plates provide magnification and shield the deflection plates from the strength of post electron acceleration field but scatter the electrons and thus hurt spot size. | Shaped metal meshes placed after the deflection plates provide magnification and shield the deflection plates from the strength of post electron acceleration field but scatter the electrons and thus hurt spot size. | ||