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Distributed deflection plates: Difference between revisions
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Distributed deflection plates (view source)
Revision as of 17:57, 12 December 2023
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==Reality== | ==Reality== | ||
As a generalization, oscilloscopes with 100 MHz bandwidth or less do not have a distributed deflection structure. (The [[580-series scopes]] with a distributed deflection structure and a 100 MHz bandwidth is an exception). | |||
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 flat ribbons wound into a helix. Through the middle of each square helix is a square metal tube that is a 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. | |||
[[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]]]] | ||