The present invention relates to an electron gun for a picture tube, such as a CRT, comprising a cathode, a first electrode (grid) and a second electrode (grid), the first and second electrodes each having an aperture for allowing passage of electrons emitted from the cathode.

The drive voltage range of an electron gun is defined as the difference between the cut-off voltage (the cathode voltage level at which no electrons are emitted from the cathode) and the cathode voltage at which an electron beam is generated with a peak beam current. This is the highest beam current that is required in the CRT.

In a typical electron gun for use in a television CPT (Color Picture Tube) the cut-off voltage is 160 V, while the cathode voltage for a peak beam current of 4 mA is around 30 V. This results in a voltage drive range of 130 V.

For a monitor CDT (Color Data Tube) the cut-off voltage is 120 V and the typical cathode voltage for a peak beam current of 0.5 mA is around 70 V. This results in a drive voltage of 50 V.

Important benefits are achieved by reducing this drive voltage range, for example by 50%, especially for high bandwidth applications such as HD TV and monitor CRTs. First, the electronic components needed to control the gun become less expensive because the video signal does not need to be amplified as much as with a conventional gun.

Also, the power dissipation in the video amplifier and the display unit in general is reduced.

This can also result in cost reductions for electronics (coolers, components, etc).

One way to lower the drive voltage range is to lower the cut-off voltage, while maintaining a sufficiently high peak beam current. These are contradictory requirements, as the cut-off voltage is lowered by decreasing the emitting area of the cathode, while the beam current is increased by increasing the emitting area of the cathode.

Geometrical changes in a conventional gun that lower the Vco are for example : making the aperture in the first grid smaller, increasing the distance between cathode and first grid, and increasing the thickness of the first grid. However, such actions

will not result in a satisfactory reduction of the drive voltage range as well as a lower peak beam current.

Attempts have been made to lower the drive voltage range by means of multiple apertures per cathode in the first grid. Instead of one single current density distribution with a high peak (1 hole in grid 1), the result is multiple current density distributions with lower peaks, which can be mixed together to form a satisfactory accumulated distribution. The multiple apertures thus increase the emitting area of the cathode and thereby the peak current. At the same time, however, each aperture has a limited area, thereby preserving or preferably lowering, the cut-off voltage.

However, this solution is not satisfactory. The most important drawback is that, as a consequence of the multiple apertures in the first grid, multiple beams will be formed in the gun for each cathode. It is difficult to achieve a good front of screen performance with multiple beams, and therefore a special gun design is needed to achieve a proper front of screen performance.

The document GB 1421865 describes a solution to achieve an astigmatic beam in a CRT. According to this solution, the first two grids are formed with elongate apertures which are oriented in the x-and y-direction, respectively. The cut-off voltage is not affected by this design. Neither is the drive voltage range.

It is an object of the present invention to provide an electron gun for a CRT, which electron gun has a drive voltage range that is significantly lower than in conventional electron guns and the peak beam current is substantially maintained and the front of screen performance deteriorates as little as possible.

This is accomplished by an electron gun as mentioned in the opening paragraph, wherein the aperture in the first electrode has a length and a width, wherein said length is greater than said width, so that said emitting area of the cathode is elongated, and the aperture in the second electrode is formed so that said apertures cooperate to define the elongated emitting area of the cathode, and leakage of electrons to the second electrode is substantially avoided.

The term"elongated"should be understood as indicating that a length of the emitting area exceeds the width of the emitting area, preferably by a ratio of at least 3: 1. The orientation of the emitting area, i. e. , if the length is oriented in the x-or y-direction, or somewhere in between, can be chosen in accordance with the specific implementation.

According to an embodiment of the invention, the apertures of the electrodes, and thus the

emitting area, are elongated in the x-direction (i. e. , narrow in the y-direction). However, in some electron gun designs it may be beneficiary to have apertures that are elongated in the y-direction.

The elongated emitting area reconciles the contradictory requirements mentioned above in an effective way. The shape of the emitting area provides sufficient peak current because the emitting area is big enough. It also makes the beam current sensitive to the drive voltage because the effect on the beam current of small changes in the current density distribution in the direction of the small dimension (e. g. , the y-direction) is amplified<BR> in the direction of the longer dimension (e. g. , the x-direction) of the emitting area. Because of this effect, a gun according to the present invention will have a lower drive voltage range than a conventional gun with the same peak current.

A secondary effect is that the small dimension (e. g. , in the y-direction) decreases the cut-off which also helps to decrease the drive range.

The preferred embodiment of the present invention will result in a drive characteristic (i. e. , relationship between cathode voltage and beam current) that is compatible with a conventional control circuit in a display unit. The shape of the apertures in the first and second electrodes can be used to alter the drive characteristic.

According to the invention, a low drive voltage is obtained by creating a relatively big emitting area with a current density distribution that is as uniform as possible.

An electron gun according to the present invention does not have the drawbacks of a multiple aperture electron gun as mentioned above. The invention provides a solution for a low drive gun with one beam per cathode with a front of screen performance comparable to a conventional gun.

Although the aperture of the first grid in GB 1421865 is elongated, the emitting area of the cathode is further limited by the shape and the orientation of the aperture in the second grid, which is of similar shape and size but oriented essentially orthogonally to the first aperture.

While the shape and the size of the emitting area is essentially determined by the shape of the aperture in the first electrode, it is the combined effect of the first and second electrode apertures that finally determines the shape of the emitting area. The width and the length of the second electrode aperture are also chosen so as not to cause a leakage current of the electron beam to the second electrode.

Preferably, the ratio between the length and the width of the aperture in the first electrode is at least 3: 1, and the length can for example be in the range from 0.7-1. 5

mm. To achieve a low drive, the length of the first electrode aperture should be as large as possible, but is limited by the mechanical limitations of the cathode. The suitable dimensions of the aperture also depend on the thickness of the first grid electrode.

More preferably, the length is in the range of 0. 8-1 mm, and most preferably around 0.9 mm. The width is preferably in the range of 0. 1-0. 4 mm, most preferably around 0. 2 mm.

In a preferred embodiment, the width of the second electrode aperture is larger than the width of the first electrode aperture, and more preferably twice as large. However, a width as small as 70% of the first electrode aperture width has been found to be satisfactory.

The length of the second electrode aperture is preferably larger than the length of the first electrode aperture, but a length as small as 70% of the first electrode aperture length has been found to be satisfactory.

Preferably, the aperture in the second electrode is also elongated and aligned essentially coaxially and in parallel with the aperture in the fist electrode. This has been found to result in a particularly advantageous shape of the emitting area.

According to one embodiment, the apertures are formed as rectangles with rounded ends, offering a simple manufacturing process.

According to a different embodiment of the invention, the aperture in the first and/or second electrode is in the shape of a dog-bone, having a larger cross section toward its ends than in the center, as seen in the direction in which the aperture is elongated.

A dog-bone shape of the first electrode aperture results in a more effective emitting area, particularly in the corners. The larger cross section ensures higher current at full drive, but should be adapted so as not to influence the cut-off voltage. A dog-bone shape of the second electrode aperture serves to reduce the lens effect at the sides of the first electrode aperture.

According to a further embodiment of the present invention, the electron gun is arranged to generate an electron beam without crossover in the longitudinal direction of the emitting area. An advantage of this is the possibility of making the electron gun shorter. This is of great importance in the attempt to CRT sets of smaller depth.

These and other aspects of the invention will be apparent from the preferred embodiments which are more clearly described with reference to the appended drawings.

Fig. 1 shows schematically in a side view an electron gun according to an embodiment of the present invention,

Fig. 2 shows the shape of the apertures in the first and second grid, in a cross section along the line A-A in Fig. 1, Fig. 3 shows an alternative design of the first electrode in Fig. 2, Fig. 4 shows the shape of the apertures in the first and second grid according to a second embodiment of the present invention, Fig. 5a shows a diagram of the current density distribution in a conventional electron gun, Fig. 5b shows a diagram of the current density distribution in an electron gun according to the invention, Fig. 6a shows a cross section along the line B-B of the triode in Fig. 1, Fig. 6b shows a magnification of the triode of Fig. 1.

Several sections can be distinguished in an electron gun. For simple guns, as the example shown in Fig. 1, the main regions are: triode 1 (i. e. a cathode 2 and two electrodes, Gl, G2), pre-focus lens 3 (i. e. the field between G2 and a third electrode G3), and main lens 4 (i. e. , the field between the third electrode G3 and a fourth electrode G4). More complicated guns include a Dynamic Astigmatism and Focus (DAF) section, and a Dynamic Beam Forming (DBF) region.

In the triode 1, which is the main concern of the present invention, there are essentially three voltage levels: the voltage of G1, Vgl, the voltage of G2, Vg2, and the voltage of the cathode 2, Vcathode.

By controlling the cathode potential (Vcathode) the field between the electrodes Gl and G2 is controlled. The flat cathode 2 emits electrons, and by placing G1 at a lower potential than the cathode 2, the electrons are prevented from landing on G1. A high positive potential at G2 accelerates the electrons.

In most electron guns, Vg2 and Vgl are fixed, typically at around Vg2=700 V and Vgl=0 V. Vcathode is varied to modulate the beam current; at a higher cathode voltage, fewer electrons are drawn into the beam by G2 and at a lower cathode voltage, more electrons are drawn into the beam. A typical variation of Vcathode in a conventional CPT is between 10 and 200 V, responding to a variation in beam current between 0 and 6 mA.

The present invention is also applicable in electron guns with a so called grid 1 drive. In such guns, Vcathode is fixed, and the beam current is regulated by varying the grid 1 voltage.

The electrons thus emitted from the cathode 2 and accelerated by Vg2 pass through apertures 10,11 formed in electrodes Gl and G2. Only electrons emitted from a limited area immediately in front of the aperture in Gl, referred to as the emitting area 12, will successfully pass through the apertures 10,11 in Gl and G2.

According to the invention, the apertures 10,11 are elongated, and can be rectangular in shape as illustrated in Fig. 2. This results in an elongated emitting area 12.

The dimensions of the apertures preferably have a ratio of at least 3: 1, i. e. , the length of the aperture is at least three times as large as the width. The aperture 11 of G2 is preferably a little longer than the aperture 10 of Gl, in order to prevent interception of the beam.

In the illustrated example, the cathode has an active (barium coated) area with a diameter of 1. 3mm, and the thickness of the electrode Gl is 0.08 mm. Mechanical positional tolerances and shrinkage of the barium layer cause the Gl aperture length to be smaller than 1.3 mm, and a length of 0.9 mm has been found suitable. Note that the Gl aperture can be made longer, but that this involves the risk that the aperture is not located above the active area of the cathode. Even so, a Gl aperture length of up to 1. 5 mm or even more is not impossible, as it is possible to limit the resulting emission by adjusting the G2 geometry.

The width of the Gl aperture can be in the range from 0.15-0. 35 mm, preferably around 0.2 mm.

Preferably the aperture 10 is made in one electrode element Gl, but said aperture can also be achieved by assembling multiple grids Gla, Glb each having an aperture 10a, lOb, such that the area of overlap 10c fulfils the length/width ratio. This is illustrated in Fig. 3.

The second electrode G2 preferably is divided into two sections 13,14 (see Figs. 6a and 6b), which means that the aperture 11 is in fact made up of two apertures of different shape and size. Again, in simulations in which the thickness of the first section 13 closest to the cathode is 0.2 mm, this section 13 can have an aperture with dimensions of 1.5 mmxO. 5 mm, i. e. , slightly larger than the aperture in the first electrode G1. The second section 14, with a thickness of 0.4 mm, can have an aperture with dimensions of 2.0 mmxl. 5 mm.

Naturally these dimensions are only given by way of example, and it is left to the skilled person to determine the optimal design in each specific implementation, within the scope of the present invention as defined by the claims. As mentioned above, the G2 aperture

cooperates with the Gl aperture to form the emitting area and to avoid leakage of current to G2. It may be advantageous if both dimensions of aperture 11 are larger than the corresponding dimension of aperture 10, but it has been determined that dimensions of aperture 11 as small as 70% of the corresponding dimensions of aperture 10 may be successful.

An alternative embodiment is shown in Fig. 4, where the aperture 111 in G2 is shaped as a dog-bone, i. e. slightly wider in each end than in the middle. This shape weakens the lens effect at the side of the Gl aperture, and therefore weakens the distortion at the sides of the beam. It may also be advantageous to make the aperture in Gl dog-bone shaped. In this case, the increased width at the edges can result in an increased current at full drive.

While one aperture is dog-bone shaped, the other aperture may be rectangular, such as aperture 110 in Fig. 4, or they may both be dog-bone shaped.

The effect of the elongated apertures 10,11 and the elongated emitting area 12 is an increase in sensitivity of the triode by creating a more uniform current density distribution compared with a conventional current density distribution.

The present invention can also be used to decrease the maximum current density, which is beneficial to the service life of the cathode.

Fig. 5a shows the current density distribution of a conventional triode, with circular apertures in Gl and G2. Fig. 5b shows the current density distribution of a triode with apertures according to the invention. It is clear from these diagrams that the inventive triode presents a more elongated electron beam with a very uniform distribution.

The narrow aperture 10 in Gl also contributes to a lower cut-off voltage. This also helps to obtain a low drive.

According to the preferred embodiments, the shape of the apertures result in a drive characteristic compatible with conventional control circuits of the display driver. This facilitates implementing the electron gun according to the invention.

When shaped according to the preferred embodiments, the elongate apertures serve to avoid crossover in the longitudinal direction. Figs. 6a and 6b illustrate a cross section taken along the lines A-A and B-B in Figs. 2 and 3, respectively. In other words, Fig.

5a is a cross section in the longitudinal direction of the elongate apertures, while Fig. 5b is a cross section in the transverse direction of the elongate apertures.

While in a conventional electron gun crossover, i. e. , the paths of the electrons emitted from the cathode cross each other, takes place, in both the x-direction and the y- <BR> <BR> direction, it is clear from Figs. 6a and 6b that crossover occurs only in one direction, i. e. , the

transverse direction of the elongated apertures. In the other direction, (in the longitudinal direction of the apertures) the electron rays do not cross each other.

According to a preferred embodiment, the apertures in Gl and G2 are oriented <BR> <BR> in the x-direction, i. e. , the longitudinal direction of the apertures is the x-direction (across the display). In a conventional electron gun, some length is required for generating crossover in all directions. The pre-focus lens behind the crossover point then ensures a favorably shaped beam. According to the above-described embodiment of the invention, no space is needed for crossover in the x-direction. In the y-direction, the thin beam ensures that not much space is necessary for generating a favorably shaped beam. All in all, this enables the overall length of the gun to be reduced.