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Distributed deflection plates (view source)

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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.  
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 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.
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.


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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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 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, which does not have a distributed deflection plate, 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 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.
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.
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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.


As a generalization, oscilloscopes with 100MHz or less bandwidth do not have a distributed deflection structure.  (The 580 series with a distributed deflection structure and a 100MHz bandwidth is an exception).  
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).  
   
   
[[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]]]]


==History==
==History==
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==Literature==
==Literature==
* [[Media:US2922074.pdf|US Patent 2,922,074, "Electron Beam Deflection Structure"]] by [[Cliff Moulton]]
* [[Cliff Moulton]], [[Patent US 2922074A|US Patent 2,922,074, ''Electron Beam Deflection Structure'']]
* {{Keller 1991| }}
* {{Keller 1991| }}
* [[Media:062-0852-01.pdf | Oscilloscope Cathode-Ray Tube Concepts]], Chuck Devere, 1969
* [[Media:062-0852-01.pdf | Oscilloscope Cathode-Ray Tube Concepts]], Chuck Devere, 1969
* {{Addis, John,  Analog Circuit Design, Art, Science, Personalities, page 115 }}
* [[John Addis]] in ''Analog Circuit Design: Art, Science, and Personalities'', page 115


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