77
edits
Distributed deflection plates: Difference between revisions
Distributed deflection plates (view source)
Revision as of 02:08, 12 December 2023
, 12 December 2023no edit summary
No edit summary |
mNo edit summary |
||
| Line 11: | Line 11: | ||
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. | ||
Making the transit time shorter by shortening | Making the transit time shorter by shortening the deflection plate increases the bandwidth, but reduces the deflection sensitivity by the same factor. Using a distributed deflection plate structure is a way around this, extending the bandwidth without reducing the deflection sensitivity. The only trade off is in complexity and therefore cost. | ||
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. | 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 keep seeing the same voltage 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 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, 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. | ||
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 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. | ||
==Reality== | ==Reality== | ||
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. | ||
As a generalization, oscilloscopes with 100 MHz bandwidth or less do not have a distributed deflection structure. (The 580 series with a distributed deflection structure and a 100 MHz bandwidth is an exception). | 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). | ||
[[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]]]] | ||