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Distributed deflection plates: Difference between revisions
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Distributed deflection plates (view source)
Revision as of 20:23, 12 December 2023
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[[File:Distributed deflection plates.jpg|300px|thumb|right|Distributed vertical deflection plates and delay lines in a [[T581]] CRT (beam direction left to right)]] | [[File:Distributed deflection plates.jpg|300px|thumb|right|Distributed vertical deflection plates and delay lines in a [[T581]] CRT (beam direction left to right)]] | ||
[[File:Distributed deflection schematic.jpg|thumb|300px|right|Simplified schematic of distributed deflection structure]] | [[File:Distributed deflection schematic.jpg|thumb|300px|right|Simplified schematic of distributed deflection structure]] | ||
In [[CRT]]s, a trade/off exists between writing rate, deflection sensitivity and spot size. Within a given technology (e.g. mono acceleration, post deflection acceleration or microchannel plate | In [[CRT]]s, a trade/off exists between writing rate, deflection sensitivity and spot size. 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 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. | ||
Meshes that shield the deflection plates from the strength of post electron acceleration field and electron lenses both trade off sensitivity against spot size to varying degrees. They do, however, contribute net performance improvements. | Meshes that shield the deflection plates from the strength of post electron acceleration field and electron lenses both trade off sensitivity against spot size to varying degrees. They do, however, contribute net performance improvements. | ||
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A real deflection plate structure is slightly limited by the fact that the last deflection plates may be longer and more widely separated than the others. Furthermore, the characteristic impedance of the deflection plate structure may not be identical to that of the leads going into and out of the CRT. These are usually high impedance lines and difficult to make. | A real deflection plate structure is slightly limited by the fact that the last deflection plates may be longer and more widely separated than the others. Furthermore, the characteristic impedance of the deflection plate structure may not be identical to that of the leads going into and out of the CRT. These are usually high impedance lines and difficult to make. | ||
Higher frequency deflection structures use more deflection plates that are closer together. Electrically they look more like transmission lines and some are traveling wave structures. For example, the 7104 deflection structure is called a box helix. The identical top and bottom deflection plates are both a | Higher frequency deflection structures use more deflection plates that are closer together. Electrically they look more like transmission lines and some are traveling wave structures. For example, the 7104 deflection structure is called a box helix. The identical top and bottom deflection plates are both constructed as a ribbon wound into a square helix. Through the middle of each helix is a square metal tube, the ground plane. Each deflection plate has a characteristic impedance of 100 ohms, which is mostly determined by the ground plane inside the helix. | ||
At high frequencies each turn of the helix electromagnetically couples with the next turn down the line. As the frequency goes up, the coupling increases causing an increase in the wave's velocity. This "velocity dispersion" causes the electron beam and the signal to be spatially separated, to the detriment of frequency response. | At high frequencies each turn of the helix electromagnetically couples with the next turn down the line, causing the signal to jump ahead slightly. As the frequency goes up, the coupling increases causing an increase in the wave's velocity. This "velocity dispersion" causes the electron beam and the signal to be spatially separated at high frequencies, to the detriment of the frequency response. | ||
[[File:Tek7844-v-b2.jpg|300px|thumb|right|Vertical termination resistor (l) and amplifier (r) in a [[7844]]]] | [[File:Tek7844-v-b2.jpg|300px|thumb|right|Vertical termination resistor (l) and amplifier (r) in a [[7844]]]] | ||