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Distributed deflection plates: Difference between revisions
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Distributed deflection plates (view source)
Revision as of 21:07, 12 December 2023
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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. | 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. | ||
The higher the required bandwidth, the more likely the distributed deflection plate structure will physically look like a uniform transmission line than separate plates connected by wires. | The higher the required bandwidth, the more likely the distributed deflection plate structure will physically look like a uniform transmission line than separate plates connected by wires. | ||
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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 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. | ||