Distributed deflection plates: Difference between revisions

Distributed deflection plates (view source)

Revision as of 16:52, 11 December 2023

88 bytes removed , 11 December 2023

m

no edit summary

Johnaddis

152

edits

@@ Line 9: / Line 9: @@
 Even the 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.
-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.  If the drive voltage is going up at the time an electron enters the deflection plate area, 1 ns later the drive voltage will be going down again, pushing the beam back to the center, i.e. 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 up the deflection plate improves 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.
@@ Line 15: / Line 15: @@
 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 equals 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.  So the electrons keep seeing the same voltage no matter where they are along the structure.
+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.  So 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. The transmission line has absorbed the plate capacitance.  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 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.
-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 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 a distributed deflection plate, has only 4 divisions of deflection because of vertical deflection plate interception.
 The higher the required bandwidth of the CRT, the more likely the distributed deflection plate structure will physically look like a uniform transmission line than separate plates connected by wires.