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Distributed deflection plates: Difference between revisions
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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. | ||
Shaped meshes provide magnification and shield the deflection plates from the strength of post electron acceleration field but scatter the electrons and thus hurt spot size. Electron lenses do not scatter electrons but | Shaped meshes provide magnification and shield the deflection plates from the strength of post electron acceleration field but scatter the electrons and thus hurt spot size. Electron lenses do not scatter electrons but nevertheless magnify spot size along with beam deflection. Both do, however, contribute net performance by isolating the deflection structure from the acceleration potential. | ||
All CRTs have a finite frequency response but distributed deflection plates extend the CRT's bandwidth as well as aid the vertical amplifier by virtually eliminating the | All CRTs have a finite frequency response but distributed deflection plates extend the CRT's bandwidth as well as aid the vertical amplifier by virtually eliminating the capacitive load on the vertical amplifier. | ||
Even a CRT with a single pair of deflection plates has frequency response not as simple as an RLC circuit. A voltage step applied to a single pair of deflection plates simultaneously affects all the electrons between the plates. Those that are just exiting the plates see nothing as they continue on their way to the phosphor for display. Those that are at the entrance to the plates feel the effects of all voltage changes that take place during their transit through the plates. Therefore they bear the memory of any deflection plate voltage changes during their journey through the plates. The effects of any deflection plate voltage changes are delayed in proportion to their distance from the exit simply because it takes time for the electrons to travel to exit of the plates. This is what makes the frequency response so complicated. | Even a CRT with a single pair of deflection plates has frequency response that is not as simple as an RLC circuit. A voltage step applied to a single pair of deflection plates simultaneously affects all the electrons between the plates. Those that are just exiting the plates see nothing as they continue on their way to the phosphor for display. Those that are at the entrance to the plates feel the effects of all voltage changes that take place during their transit through the plates. Therefore they bear the memory of any deflection plate voltage changes during their journey through the plates. The effects of any deflection plate voltage changes are delayed in proportion to their distance from the exit simply because it takes time for the electrons to travel to exit of the plates. This is what makes the frequency response so complicated. | ||
For example, if it takes 1 ns for an electron to travel the length of the plates (usually one on each side of the electron beam) and a 1 GHz sine wave is applied between the plates, the full 360 degrees of the sine wave causes the electron to move up and down during transit. By the time the electron exits the plate area 1 ns later, the electron is back where it started from. There is no net deflection and the sensitivity is zero at this frequency. | For example, if it takes 1 ns for an electron to travel the length of the plates (usually one on each side of the electron beam) and a 1 GHz sine wave is applied between the plates, the full 360 degrees of the sine wave causes the electron to move up and down during transit. By the time the electron exits the plate area 1 ns later, the electron is back where it started from. There is no net deflection and the sensitivity is zero at this frequency. | ||
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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. | 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, 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. | 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]]]] | ||