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Distributed deflection plates: Difference between revisions
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Distributed deflection plates (view source)
Revision as of 13:59, 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 (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 | 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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In the distributed deflection plate structure, the original deflection plates are cut up into individual segments. The capacitance of the deflection plates can then be made part of a lumped delay line by adding inductance between each of the plate segments. These inductors are actually inside the CRT. | In the distributed deflection plate structure, the original deflection plates are cut up into individual segments. The capacitance of the deflection plates can then be made part of a lumped delay line by adding inductance between each of the plate segments. These inductors are actually inside the CRT. | ||
Signals in any lumped delay line travel relatively slowly. If done correctly, the electron beam velocity can equal the signal velocity down the lumped delay line. The signal and electrons travel together. This reduces the time the electron beam spends between any particular deflection plate pair by the number of plates. But the electrons | Signals in any lumped delay line travel relatively slowly. If done correctly, the electron beam velocity can equal the signal velocity down the lumped delay line. The signal and electrons travel together. This reduces the time the electron beam spends between any particular deflection plate pair by the number of plates. But the electrons react to the same electric field no matter where they are along the structure. | ||
The lumped delay line is terminated at the end of the deflection structure outside the CRT. The end of this delay line needs to be terminated to prevent the drive signal from being reflected back through the line. In Tektronix scopes, the termination resistor can often be seen attached to a second pair of vertical deflection terminals on the side of the CRT, which bring out the end of the transmission line. This means the vertical amplifier is driving a resistive load and not the capacitance of a long deflection plate. In some scopes (e.g. the [[7104]]), even the output impedance of the vertical amplifier is equal to the CRT characteristic impedance. While this is not essential, it does reduce incidental reflections. | The lumped delay line is terminated at the end of the deflection structure outside the CRT. The end of this delay line needs to be terminated to prevent the drive signal from being reflected back through the line. In Tektronix scopes, the termination resistor can often be seen attached to a second pair of vertical deflection terminals on the side of the CRT, which bring out the end of the transmission line. This means the vertical amplifier is driving a resistive load and not the capacitance of a long deflection plate. In some scopes (e.g. the [[7104]]), even the output impedance of the vertical amplifier is equal to the CRT characteristic impedance. While this is not essential, it does reduce incidental reflections. | ||
The termination resistor is outside the CRT because the transient response is cleanest when the termination is adjustable, and a laser trimmed resistor inside the tube would contaminate the tube's vacuum. | |||
Note that the last deflection plates are often tilted and farther apart than the others. This is to prevent the deflection plates from intercepting and cutting off the electron beam at large deflections. The famous 545A's CRT, which does not have distributed deflection plates, has only 4 divisions of deflection because of vertical deflection plate interception. | Note that the last deflection plates are often tilted and farther apart than the others. This is to prevent the deflection plates from intercepting and cutting off the electron beam at large deflections. The famous 545A's CRT, which does not have distributed deflection plates, has only 4 divisions of deflection because of vertical deflection plate interception. | ||