Dynamic focussing circuit for cathode ray tube and transformer for use therein

Abstract: A diode modulation circuit provides a horizontal yoke coil current which is modulated in a manner to correct for side pin cushion distortion in the resultant CRT television picture tube display. The modulation circuit includes two parallel capacitance-inductance resonant circuits coupled in series. One coil of the parallel resonant circuits is the horizontal yoke coil of the picture tube. A modulation current is applied to the coil of the other parallel resonant circuit to generate a modulation current in the yoke coil. The modulation current flowing through the yoke coil is applied to the primary winding. Another modulated current in the diode modulator circuit is applied to a secondary winding of a transformer, the windings being coupled to each other via a common core. The turns ratio of the primary winding to the secondary winding of the transformer are such as to add the fluxes generated by the windings to produce a net flux in the transformer core that exhibits negligible modulation. A ramp current of constant maximum amplitude is induced in the output winding of the transformer by that flux and is applied to an output capacitor circuit for producing an unmodulated periodic parabolic voltage for driving the focusing grid of the CRT.

Description: This invention relates to a dynamic focusing circuit as defined in the preamble of claim 1. The invention further relates to a picture display apparatus as defined in the preamble of claim 9. In a cathode ray tube display device, the radius of deflection of the electron beam generating the display on the display screen of the tube is generally not the same as the radius of curvature of the display screen. Presently, the display screen of cathode ray tubes (CRT) are becoming generally more planar thus having a relatively large radius of curvature relative to that of the electron beam. Typically, the trend today is to make the display tube as short as possible, therefore, reducing the radius of the electron beam while at the same time the trend is to flatten the display screen and therefore increasing its radius. Further, screen size, that is overall dimensions of the display are also being made increasingly larger. As a result, the disparity between the radius of the electron beam and that of the display screen is becoming more significant. Because of these differences in radii, the electron beam is required to travel a much greater path to the edges of the screen than it does to the centre of the screen. The electrons in an electron beam, as known, tend to spread proportionally with the distance travelled resulting in a defocusing effect at the screen edges. The defocusing effect therefore becomes emphasized as the relative size or flatness of the screen increases and as the beam radius of deflection decreases. To correct this defocusing problem, a voltage which varies with the angle of deflection is added to the static focus voltage applied to the electron beam. This correction voltage is known as a dynamic focusing voltage. The defocusing phenomena is a function of the square of the distance from the centre of deflection of the electron beam. Consequently, the dynamic correction voltage is generally parabolic in shape, with increasing magnitude of the voltage occurring at the edges of the screen in order to provide more uniform focusing of the beam on the screen. Thus for every scan line, the parabolic voltage applied to the scan line signal has parabolically increasing magnitudes at the ends of each scan line and parabolically decreasing magnitude at the centre of the scan line. The scan line is generated by a scan line ramp current applied to the horizontal yoke coil. Circuits for generating such parabolic focusing voltages are shown in Figures 1a and 1b respectively illustrating voltage driven and current driven circuits. A diode modulator is utilized in television receivers with relative large, for example 110°, deflection angles for correction of side pin cushion raster distortion. A description of one such diode modulator is given in U.S. patent US-A-3,906,305. The side pin cushion raster distortion when observed uncorrected, causes the raster to have an hour glass shape. That is, the scan lines in the centre of the screen are shorter in length than the scan lines at the top and bottom of the screen. The diode modulator circuit corrects for this distortion by increasing the horizontal yoke current at the centre of the raster and decreasing the horizontal yoke current at the top and bottom edges of the display on the screen. Such a circuit, for example is illustrated in Figure 2 herein which will be described below in the detailed description. The modulation of the yoke horizontal current results in the horizontal yoke current waveform having a vertical rate (60 Hz) modulation. The horizontal yoke coil deflection beam ramp current applied to the horizontal yoke coil is utilized in accordance with the embodiment of Figure 1b to generate a dynamic focus voltage. However, because that yoke coil ramp current is modulated, this modulation tends to be carried through to the dynamic focusing voltage generated therefrom providing a modulated periodic parabolic focusing grid voltage. This modulation of the dynamic focusing voltage is undesirable. Because of this modulation of the focusing voltage, the electrons of the electron beam will not be uniformly focused on the screen at the edges of the display in the different scan lines causing a variable focusing effect at the screen edges. Such a defocusing of the display is undesirable. JP-A-3 053 671 discloses a diode modulator for generating a modulated horizontal yoke current to correct for pincushion distortion on the display screen of the tube, and a dynamic focusing voltage generating circuit being coupled to the diode modulator. The dynamic focusing voltage generating circuit is responsive to the modulated horizontal yoke current, and thus supplies a dynamic focusing voltage being modulated by the same pincushion distortion component. It is, inter alia, an object of the invention to provide an optimal dynamic focusing voltage. To this end a first aspect of the invention provides a dynamic focusing circuit as defined in claim 1. A second aspect of the invention provides a picture display apparatus as defined in claim 9. Advantageous embodiments are defined in the sub claims. A circuit for producing a dynamic focusing voltage for a cathode ray tube raster scan beam deflection yoke, the tube having a face for receiving the scan beam for generating a raster scan picture on the face in response to the beam incident thereon according to an embodiment of the present invention comprises a diode modulation circuit for modulating a beam deflection current applied to the yoke for scanning the beam on the face of the tube to correct pin cushion distortion in the picture. Means are responsive to the modulated current for producing a substantially unmodulated beam focusing voltage from the modulated current. In accordance with one embodiment of the present invention, the means for producing a substantially unmodulated beam focusing voltage include a transformer having a core, a first winding coupled to the core for receiving the modulator current, and second and third windings coupled to the core and first winding via the core, and capacitive means coupled to the third winding for generating a periodic dynamic focusing voltage in accordance with the value of flux produced in the core by the first and second windings. The first and second windings have a relative turns ratio such that the flux in the core coupled to the third winding remains substantially constant regardless of the presence of modulation of the modulated current. The foregoing and other aspects of the invention are described in more detail by way of example with reference to the accompanying drawings. In the drawings: Figures 1a and 1b are representative prior art circuits of respective voltage driven and current driven dynamic grid focus voltage generating circuits; Figure 2 is a circuit diagram of a prior art diode modulating circuit for modulating the horizontal yoke coil scan line current to correct for pin cushion distortion; Figures 3a, 3b, 3c and 3d illustrate waveforms produced at different points in the circuit of Figure 2; Figure 4 is a circuit diagram of a diode modulation and dynamic focusing grid voltage generating circuit in accordance with one embodiment of the present invention; and Figure 5 illustrates a waveform at the output of the circuit of Figure 4. In the different figures, components with the same reference numerals are identical. In Figure 1a, a prior art voltage driven circuit for generating a periodic parabolic focusing waveform includes a horizontal yoke coil L h which receives a ramp scan current I H from a source (not shown) in a typical television receiver. The coil L h is the horizontal yoke coil, for example, in a cathode ray tube used as a display in a television receiver. Each ramp of the current I H is used to generate a scan line of the deflected electron beam generating the display in the CRT. The current outputted from the coil L h is applied to a reference potential through a capacitor C S . A periodic parabolic voltage waveform is generated across the capacitor C S . A transformer T1 has a primary winding 10 which is connected in series with a DC blocking capacitor C B across capacitor C S . The secondary winding 12 of the transformer has one end coupled to ground and the other end coupled to the CRT grid circuit which receives the dynamic focusing voltage through DC blocking capacitor C p . C p typically has a rating value of 15 kv and applies the periodic parabolic waveform to the cathode ray tube grid for focusing the scanning electron beam. In Figure 1b, a transformer T1' has its primary winding 10' connected to one side of capacitor C S and the other side to ground. The secondary winding 12' has one end connected to ground. A parabolic waveform generating capacitor C S1 is coupled across the winding 12'. Capacitor C p which couples the periodic parabolic voltage is connected between the junction of winding 12' and capacitor C S1 and the output of the circuit. In operation of the circuit of Figure 1b by way of example, the ramp current I H is applied to the horizontal yoke coil L h through capacitance C S and winding 10' to ground. Transformer T1' steps down the current in winding 10' to produce a reduced ramp current in winding 12'. Capacitor C S1 produces a parabolic waveform from the ramp current in winding 12'. DC blocking capacitor C p passes the periodic parabolic focusing voltage to the CRT grid. Typically, the current I H is a sawtooth current of a given maximum amplitude. However, as mentioned in the introductory portion, the resultant amplitude ramp current for creating the horizontal scan lines results in the pin cushion effect in the display such that the scan lines are foreshortened in the centre portion of the display screen. To correct for this pin cushion effect caused by typical yoke designs, the diode modulating circuit of Figure 2 is provided as described in more detail in U.S. Patent 3,906,305. The circuit 20 of Figure 2 includes an NPN transistor 21 which operates like a switch and together with coil L P coupled to a voltage source B+, and coil L h produce current I H . For example, transformer T1 in this embodiment switches at 15,734 Hz (the horizontal frequency) or every 63.55 µsec. The emitter is coupled to ground and the collector is connected to the junction of parallel resonant circuit capacitor C r , coil L p and horizontal yoke coil L h . Diode 22 has its cathode connected to the junction of capacitor C r and coil L h and its anode connected to the junction of capacitance C r and a second capacitor C b . A second diode 24 has its cathode connected to the anode of diode 22 and its anode connected to the other side of capacitor C b . The coil L h is connected to one end of the primary winding 26 of current step up transformer T1'' through a DC blocking coupling capacitor C S . The other end of the winding 26 is connected to the junction of diodes 22 and 24 and capacitors C r and C b . The junction of the winding 26 with the diodes 22 and 24 is connected to ground through an inductance L b serially connected with coupling capacitor C S '. Transformer T 1 '' has a core 28 to which is coupled a secondary winding 30. The secondary winding is coupled to output 32 in a current driven configuration as illustrated in Figure 1b. In this configuration a capacitor 34 is connected across winding 30 to generate the periodic parabolic waveform and output coupling capacitance 36 applies it to the focusing grid voltage on output 32. An injection coil 38 receives an incoming signal to inject a modulating waveform to the capacitor C S '. This waveform modulates the current I H applied to the horizontal yoke coil L h . Modulation of the current applied to the coil L h is of a direction and magnitude so as to correct for the pin cushion distortion mentioned in the introductory portion. The horizontal dynamic focus parabolic voltage thus generated has a vertical rate (60 Hz) modulation because of the modulation of the horizontal yoke current. Figure 3a illustrates the modulation of the ramp current I H waveform a, the modulation envelope being illustrated by the dashed lines. The ramp current I H for generating the scan lines is illustrated by the solid lines. The scan lines are modulated so that the modulation amplitude A varies in a periodic manner as shown. This current is applied to the primary winding 26 of transformer T1'' producing the waveform c, Figure 3c, in winding 26. The modulated current for winding 26 induces a modulated current in the winding 30 of transformer T1''. Modulation of the current in winding 30 produces a modulated periodic parabolic voltage waveform across capacitor 36 as illustrated in Figure 3d. The amplitude of each of the individual parabolas of the waveform in a given period differs from one another as shown. This voltage when applied to the focusing grid of the CRT results in undesirable defocusing of the display at the display edges as previously described. In Figure 2, the coil L b and capacitor C b form a second parallel resonant circuit tuned to the same frequency as the parallel resonant circuit formed by capacitor C r and coil L h , the two resonant circuits being in series. Current I B in coil L b is subtracted from the current I H in coil L h to produce a modulation current I m . Current I H is lower in value at the top and bottom of the display screen than at the centre of the screen. In contrast, the current I B has the opposite characteristic. Current I B has a smaller value at the centre of the screen than it has at the screen edges. Also, the degree of modulation of the current I B is greater than that of the modulation of the current I H . This is illustrated by comparison of Figures 3a and 3b. The modulation frequency of the waveforms of 3a-3d is 1/f v where f is the frequency of the vertical scan rate, which typically is 60 hertz. Modulation of the grid focusing voltage waveform d, Figure 3d, is removed by the circuit in accordance with one embodiment of the invention illustrated in Figure 4. In Figure 4, the two parallel circuits comprising capacitor C r and coil L h forming one resonant circuit and capacitor C b and coil L b forming a second resonant circuit in series with the first circuit are the same as shown in Figure 2. However, a transformer T2 is used in place of the transformer T1'' of Figure 2. The transformer T2 has a primary winding L1 connected in series with coupling capacitor C S and coil L h . Capacitor C S ' connected to the other side of coil L b is connected to ground potential through a second winding L2 of the transformer T2. The windings L1 and L2 are coupled via core 40 of the transformer T2. In this arrangement the current I B from coil L b and capacitor C S ' flows through the winding L2 to ground. The turns ratio of windings L1 and L2 is significant. The current flowing in winding L1 flows in a direction opposite the current flowing through winding L2 relative to the core 40. The turns ratio of the winding L1 to winding L2 is set, such that the flux created in the core 40 by these currents remains constant in the presence of the modulation of the current through winding L1 and through winding L2. The modulation of the current applied to winding L1 produces a modulation of the flux in core 40. However, the modulating current applied to winding L2 produces a flux that is added to the modulation of the flux created by winding L1. The relative magnitude of these fluxes is determined by the turns ratio of the two windings such that the modulation of one winding flux added to the modulating flux created by the other winding is a constant value. By adding the current I B to the current I H via the transformer T2, the resultant net flux has relatively negligible modulation. In this case, the degree of modulation of the current I B is greater than that of current I H . Current I B is therefore reduced until it equals the vertical rate modulation of I H . By transformer T2 combining the two waveforms I B and I H , a ramp current is produced in winding L S that is unmodulated. Thus, the currents I H and I B are combined in transformer T2 magnetically via the flux in core 40. The flux is equal to the sum of the algebraic currents combined proportionally. Therefore, the turns of winding L1 and of winding L2 are set in accordance with the value of the current supplied thereto to algebraically add the flux modulations created by each of the windings so as to effectively produce a net constant maximum amplitude periodically varying flux. For example, in one implementation, the winding L1 may comprise 12 turns as compared to winding L2 having 4 turns. However, the ratio of the modulation amplitude of the current in winding L1 to the modulation amplitude of the current in winding L2 is assumed 1:3. The current in winding L1 may have a value of 6.0 to 6.5 A and the current in winding L2 may have a value of 1.35 to 3.75 A. The windings are made of the same wire. The actual turns ratio between windings L1 and L2 is determined by the amplitude modulation ratio between the currents I H and I B applied respectively to the windings L1 and L2. The amplitude modulation of current I B is greater than that of current I H , thus more turns are provided the winding L1 receiving current I H , as compared to the number of turns in winding L2 receiving current I B . The flux in the core of transformer T2 is determined by the ampere turns. This is determined by n x I, where n is the number of turns and I is the current flowing through the turns. Adding the ampere turns of each of the windings produces the total net flux in the core. Making the flux in the core 40 relatively constant and removing the modulation, the waveform across the output winding L S of transformer T2 is a periodic ramp current of constant maximum amplitude. This current results in a periodic output parabolic waveform without modulation as illustrated in Figure 5, waveform g. A resistance R d is connected to the junction of capacitor C S and winding L1 and the junction of diodes 22, 24 and windings L1, L b . This resistance reduces the Q of the tuned circuit to reduce ringing and oscillation in the circuit.

Claims: A dynamic focusing circuit for generating a focusing voltage (g) for a cathode ray tube, said focusing circuit comprising: scan means (Cr,Cb,Lh,Lb,38) for producing a modulated scan current (IH) to be applied to a deflection yoke (Lh) for scanning a raster on a screen of the cathode ray tube, said modulated scan current (IH) having an amplitude envelope being periodically modulated to correct pin cushion distortion of the raster, and dynamic focusing voltage generating means (T2,C1) responsive to said modulated scan current (IH), characterized in that said dynamic focusing voltage generating means (T2,C1) are adapted to generate a substantially unmodulated periodic parabolic dynamic focusing voltage (g) in response to said modulated scan current (IH). The dynamic focusing circuit of claim 1, characterized in that the dynamic focusing voltage generating means (T2,C1) comprise: a transformer (T2) including a core (40), a first (L1), second (L2), and third (L5) winding being coupled to the core (40), the first winding (L1) being coupled to receive the modulated scan current (IH), the first (L1) and second (L2) windings having a turns ratio such that a flux in said core (40) is not modulated by the periodic modulation of the modulated scan current (IH), and capacitance means (C1) coupled to said third winding (L5) for generating the periodic dynamic focusing voltage (g) in accordance with the value of flux produced in the core (40) by the first (L1) and second (L2) windings. The dynamic focusing circuit of claim 2, characterized in that the dynamic focusing circuit further comprises coupling means (Cs) coupled between the first winding (L1) and the scan means (Cr,Cb,Lh,Lb,38) to operate the dynamic focusing circuit in a current driven mode. The dynamic focusing circuit of claim 2, characterized in that the dynamic focusing circuit further comprises damping means (Rd) coupled across the first winding (L1). The dynamic focusing circuit of claim 1, characterized in that the deflection yoke (Lh) includes a line deflection yoke (Lh), and in that the scan means (Cr,Cb,Lh,Lb,38) include a diode modulation circuit (Cr,Cb,Lh,Lb,38) for generating a modulated line scan current (IH) in the line deflection yoke (Lh) for scanning lines forming the raster, the diode modulation circuit (Cr,Cb,Lh,Lb,38) comprising: a first parallel resonant circuit including the line deflection yoke (Lh) in parallel with a first capacitance (Cr), a second parallel resonant circuit including an inductance (Lb) in parallel with a second capacitance (Cb), the first resonant circuit (Lh,Cr) being coupled in series with the second parallel resonant (Lb,Cb) circuit, the first and second resonant circuits having substantially the same resonance frequency value, and means (38) being coupled to the second parallel resonant circuit (Lb,Cb) for receiving a modulation signal for generating the modulated line scan current (IH) through the deflection yoke (Lh) to effect the pin cushion correction. The dynamic focusing circuit of claim 5, characterized in that the modulated line scan current (IH) is a first modulated sawtooth current having a substantial sawtooth shape and being amplitude modulated with a first modulation value, and in that the dynamic focusing voltage generating means (T2,C1) comprise a transformer (T2) including: a core (40), a first winding (L1) coupled to the core (40) for receiving the first modulated sawtooth current (IH), a second winding (L2) coupled to the core (40) for receiving a second modulated sawtooth current having a substantial sawtooth shape and being amplitude modulated with a second modulation value, and a third winding (L5) coupled to the core (40) for producing a substantially sawtooth shaped output current corresponding to the algebraic sum of fluxes generated in said core (40) by said first (L1) and second (L2) windings, said first (L1) and second (L2) windings having a turns ratio such that the first and the second modulation values cancel each other for obtaining the sawtooth shaped output current being non modulated. The dynamic focusing circuit of claim 6, characterized in that the first winding (L1) is coupled between the deflection yoke (Lh) and the inductance (Lb), the inductance (Lb) being coupled between the first winding (L1) and the second winding (L2). The dynamic focusing circuit of claim 6, characterized in that the dynamic focusing circuit further comprises a coupling capacitor (Cs') being arranged between the first (L1) and second (L2) winding, a free end of the second winding (L2) being connected to a reference potential. A picture display apparatus comprising: a cathode ray tube having a display screen, a deflection yoke (Lh) for deflecting an electron beam in the cathode ray tube, a dynamic focusing circuit for generating a focusing voltage (g) being supplied to the cathode ray tube, said focusing circuit comprising: scan means (Cr,Cb,Lh,Lb,38) for producing a modulated scan current (IH) to be applied to a deflection yoke (Lh) for scanning a raster on a screen of the cathode ray tube, said modulated scan current (IH) having an amplitude envelope being periodically modulated to correct pin cushion distortion of the raster, and dynamic focusing voltage generating means (T2,C1) responsive to said modulated scan current (IH), characterized in that the dynamic focusing voltage generating means (T2,C1) are adapted to generate a substantially unmodulated periodic parabolic dynamic focusing voltage (g) in response to said modulated scan current (IH).

Inventors: GOLIK STEPHEN, GOLIK, STEPHEN, Golik, Stephen, c/o Int. Octrooibureau B.V.
Applicants: PHILIPS ELECTRONICS NV, PHILIPS ELECTRONICS N.V.
Application year: 1992
Main classification: H04N3
IPC codes: H04N5, H04N3