US3439219A - Beam control circuit - Google Patents

Description

Sheet 012 C. GREENBLUM BEAM CONTROL CIRCUIT AN: mom-30m April 15, 1969 Filed March 6, 1967 lNVENTOR CARL GREENBLUM BY ATTORNEY April 15, 1969 c. GREENBLUM BEAM CONTROL CIRCUIT Sheet Filed March 6, 1967 T t l i T 2 2 2,

96 E zmohmzi $5135 $856 was SE 3,439,219 BEAM CONTROL CIRCUIT Carl Greenblum, Stamford, Conrn, assignor to The Bunker-Ram!) Corporation, Stamford, Conn., a corporation of Delaware Filed Mar. 6, 1967, Ser. No. 626,910 Int. Cl. Htllj 29/70 US. Cl. 315-26 13 "Claims ABSTRACT OF THE DISCLOSURE transitions to eliminate instability.

This invention relates to a circuit for effecting desired movement of an electromagnetically-controlled beam and, more particularly, to a circuit which permits such a beam to be simultaneously responsive to signals operating in vastly different frequency ranges while utilizing a single deflection yoke.

One common situation in which an electromagneticallycontrolled beam may be simultaneously driven by signals having radically different frequencies occurs when a. cathode ray tube (CRT) is being used to provide an alphanumeric display. A system of this type is, for example, shown in copending application, Ser. No. 460,117, entitled Data Handling Apparatus, filed June 1, 1965, on behalf of R. D. Belcher et al., and assigned to the assignee of the instant application. In these systems a relatively low frequency step wave is applied to the vertical deflection circuit of the tube to control the position at which each line sweep of the display begins. The vertical control circuit also has a relatively high frequency sawtooth wave applied to it which causes the succeeding strokes of each sweep to be generated. This means that the vertical deflection circuit must handle both a low frequency stepwave input, which may operate in a range of c.p.s. or less, and a relatively high frequency stroke signal which may vary in the 10 c.p.s. range with higher harmonies. The signal frequency components which the vertical deflection circuit is called upon to handle may therefore vary by factors of 10 40 or more.

The standard solution to the above problem has been to provide a special tube which uses a conventional yoke for the major positioning in response to the low frequency signal and, in addition, contains either a special set of diddle or electrostatic deflection plates, or a separate tweeter yoke, for developing the high frequency stroke signal. A serious disadvantage of either of the above approaches is that, since a special tube must be provided, the cost is substantial. A second, and perhaps equally serious, disadvantage is that the initial deflection by the yoke moves the beam away from the center of the tube, so that the deflection by the second yoke, or the diddle plates, is non-uniform, being influenced more by the near side to the beam and less by the far side, resulting in distortion of the projected image. In effect, the beam is being deflected from two separate origins.

For the above reasons, it is advantageous to use a single deflection yoke to provide the required response to both the high frequency and low frequency deflection inputs.

One way in which this may be accomplished is to logically combine the signals in an amplifier or similar device before applying the signals to the yoke. To do this, however, requires that the amplifier employed have a very wide frequency response with minimal distortion over the range. Amplifiers of this type are difficult to obtain and are, for this reason, expensive. Other attempts to utilize a single yoke with deflection inputs of radically different frequencies have been frustrated by signal interference and distortion resulting from a variety of other causes.

It is therefore a primary object of this invention to provide an improved deflection circuit for an electromagnetically-controlled beam.

A more specific object of this invention is to provide a circuit which is capable of deflecting an electromagnetically-controlled beam in response to input signals of vastly different frequencies without requiring special tubes or expensive additional equipment and without introducing any significant distortion into the system.

In accordance with these objects this invention provides a circuit for effecting desired movement of an electromagnetically-controlled beam which circuit has a first input signal applied to it at a frequency fl and a second input signal applied to it at a frequency 2, where f2 may be many times greater than f1. The signal at frequency f1 and the signal at frequency f2 are each coupled through a separate coupling circuit to a single beam-control deflection yoke. The coupling circuit for coupling the signal at frequency fl to the deflection yoke includes a filter for blocking passage in either direction of signals at frequency f2 and the circuit for coupling the signal at frequency f2 includes a filter for blocking passage in either direction of a signal at frequency f1. The coupling means for the signal at frequency f2 also includes a means for causing the deflection yoke to be cyclically charged at a frequency f2 and a discharge circuit for the yoke. Transitions between the charging and discharging of the yoke are smoothed to prevent distortion by providing a rectifying element which is substantially back-biased during the charging intervals and conducting during the discharge intervals, but Whose response characteristics are such that it does not immediately become cut-off during the charging interval or, in other words, has certain storage characteristics.

The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of a preferred embodiment of the invention, as illustrated in the accom panying drawings.

In the drawings:

FIG. 1 is a semi-block schematic diagram of a preferred embodiment of the invention.

FIG. 2 is a waveform diagram illustrating the signals appearing at various points in the circuit of FIG. 1.

FIG. 3 is a diagram illustrating a display pattern which may be achieved by use of the circuit of this invention.

Referring first to FIG. 3, it is seen that the display format for a CRT display system utilizing the preferred embodiment of the invention includes a number of parallel lines (4 in the figure) with a large number of sawtooth writing strokes for each lines. The vertical starting position for each line of the display is determined by the potential of a step wave input. The time required to write a line is approximately one millisecond. Therefore, the step wave frequency is approximately 10 c.p.s. The sawtooth writing strokes are achieved by applying a high frequency pulse wave to the vertical deflection control circuit. Assuming one thousand strokes to the line, which is a reasonable assumption, the frequency of the pulse Wave would be 10 c.p.s. The advancing of the strokes across the line, which advancing forms no part of the present invention, is achieved by applying a low frequency sawtooth to a horizontal deflection control circuit. Character writing is achieved by selectively energizing the beam as it traces the above described raster pattern on the face of CRT 8. An overall system for achieving a display of the type described above is described in detail in the before-mentioned Belcher et al., application.

FIG. 1 shows, in detail, the preferred embodiment of a vertical deflection control circuit suitable for use in the above-described system. A staircase waveform at a frequency fl (for example, c.p.s.) is applied by a staircase wave generator 10 through a line 12 to an amplifier 14. Amplifier 14, which has an extremely low frequency response, causes the input on line 12 to appear as a low impedance driving source. The output from amplifier 14 on line 16 is applied through coupling capacitor 18 and inductance coil 20 to input terminal 22 of vertical deflection yoke 24. The capacitance of apaitor 18 is very large, thereby preserving the low frequency response of this leg of the circuit, while, at the same time, eliminating D-C response. While the inclusion of capacitor 18 is optional, the elimination of D-C response does make the line positions on the face of CRT 8 (FIG. 3) impervious to drift due to power supply variations, temperature, etc. Coil 20 serves as a filter, transmitting the low frequency position signals from generator 10 while preventing the high frequency signals, to be presently described, from coupling back to disturb amplifier 14. Line A of FIG. 2 is an illustration of the staircase wave component of the signal applied to terminal 22. From this figure it is seen that the duration of each step of the wave is (l/fl) seconds and that, for a four-line display, there are four steps before the cycle repeats. An increase in the deflection current causes the beam to move down on the face of tube 8.

Referring again to FIG. 1, it is seen that pulses at a frequency f2 from a pulse source 26 are applied to the primary 28F of a transformer 28. Source 26 may, for example, be a D-C source in series with a synchronous periodically-operated switch. Transformer 28 steps up the voltage of the pulses from source 26 to a very high amplitude. The voltage pulses are converted into constant current pulses by resistor 30 whose value is such that, in combination with the inductance L of yoke 24, the time constant (L/R) is small when compared with the sawtooth pulse width (1/ f2) The pulses from resistor 30 pass through capacitor 32 before reaching terminal 22. Capacitor 32 is of such a value as to be an effective closed circuit to the high frequency pulses such as those from transformer 28 while being is effect an open circuit to the low frequency step signals emanating from amplifier 14. In other words, capacitor 32 serves as a high-frequency filter passing the high-frequency stroke generating pulses to yoke 24, while blocking the passage of the staircase voltages back to pulse source 26.

The pulses passing from secondary coil 288 of transformer 28 through resistor 30 and capacitor 32 to coil 24 are shown on line B of FIG. 2. These constant current pulses serve to charge yoke 24 to a maximum stroke level (related to the character stroke height shown in FIG. 3). During the charging time the current through the coil increases, as shown on line C of FIG. 2, causing the beam on the face of CRT 8 to move down from the vertical position determined by the staircase waveform. During the charging of the yoke, diode 34 is substantially back-biased, and therefore open. After the charging pulse is over, yoke 24 discharges through the circuit composed of capacitor 32, diode 34, and resistor 36 to ground. As indicated previously, the inductive reactance of coil 20 is large enough to make it an effective open circuit to the high-frequency discharge current. Since the capacitance of capacitor 32 is chosen so as to be a closed circuit to signals at frequency 2, and higher harmonics thereof, the discharge time constant is established by the forward resistance of diode 34 in combination with the resistance of resistor 36. Since this resistance is low, the time constant between it and the yoke inductance is such that the current discharge is very linear, yielding excellent vertical linearity of the display strokes. The resistor 36 is optional but should generally be included to aid in making more uniform the discharged time constant by reducing the effect of variations in the forward resistance of the diodes employed.

One problem which is encountered when using a circuit of the type shown in FIG. 1 is instability in the output waveform resulting from the sharp transitions between the charging and discharging of the yoke. This instability decreases the quality of the display which may be obtained on the face of CRT 8, often obliterating one or more of the index positions at the beginning or end of a stroke, and may become serious enough so as to render the system totally inoperative. It has been found that this instability may be effectively eliminated by selecting a diode 34 which has a certain amount of storage time so that it does not become immediately cut off when a pulse is applied through resistor 30. If the response time of the diode is such that it does not become fully cut off during the initial portion of the pulse interval, the diode serves to effectively damp the instability oscillations, particularly those at the discharge-charge junction, resulting in a charge-and-discharge pattern across the yoke which is substantially that shown on line C of FIG. 2. The actual current waveform which appears across yoke 24, which is a combination of the waveforms shown on lines A and C of FIG. 2, is shown on line D of this figure.

While the values selected for the various elements shown in FIG. 1 may vary over a fairly wide range, the following set of values has been found to give satisfactory results:

f1=10 c.p.s.

Voltage across coil 28S=300 volts Yoke current=25 amps. with 10% from source 10 and from source 26 Yoke inductance=650 microhenry Inductance 20:1-2 millihenry Capacitance 18: l2 10 microfarad Capacitance 32:0.01 microfarad Resistor 30:2.5K ohms Transformer 28 is a pulse transformer with open circuit inductance=75 microhenry Diode 35 is an IN4385 specially selected to have a storage time of about 1 1O second As indicated above, the special selection of the diode 34 to have the requisite storage time is important in order to achieve stable operations of the circuit. In the alternative, a biasing circuit may be placed in series with the diode in order to alter the effective capacitance of the diode and to some extent achieve the desired storage effect.

While the invention has been particularly shown and described with reference to a preferred embodiment thereof, it will be understood by those skilled in the art that the foregoing and other changes of form and details may be made therein without departing from the spirit and scope of the invention.

What is claimed is:

1. A circuit for effecting desired movement of an electro-magnetically controlled beam comprising:

a beam-control deflection yoke;

a first input signal at a frequency fl;

a second input signal at a frequency 2, f2 being significantly greater than fl;

first means for coupling said first input signal to said deflection yoke, said first coupling means including means for effectively blocking the passage therethrough in either direction of a signal at frequency f2; and

second means for coupling said second input signal to said deflection yoke, said second coupling means including means for blocking the passage therethrough in either direction of a signal at a frequency fl.

2. A circuit of the type described in claim 1 wherein said second coupling means includes means for cyclically charging and discharging said deflection yoke at a frequency f2.

3. A circuit of the type described in claim 2 wherein said charging and discharging means includes means for smoothing the transitions between charging and discharging to prevent distortion in the beam movement.

4. A circuit of the type described in claim 3 wherein said charging and discharging means includes:

a rectifying element; and

means for biasing said rectifying element to conduct during time intervals when said deflection yoke is discharging and to be substantially back-biased during time intervals when said yoke is charging;

the response characteristics of the rectifying element being such that it does not immediately become fully back-biased during the yoke-charging intervals, thereby serving to smooth charge and discharge transitions.

5. A circuit of the type described in claim 4 wherein said rectifying element is a diode.

6. A circuit of the type described in claim 1 wherein said electromagnetically-controlled beam is the writing beam of a cathode ray tube.

7. A circuit of the type described in claim 1 wherein said deflection yoke has an input terminal; and

wherein said first and said second coupling means both apply their respective input signals to said input terminal, whereby the inputs from said first and second coupling means are summed in said deflector yoke.

8. A circuit of the type described in claim 1 wherein said second coupling means includes means for applying a relatively high-voltage, constant-current pulse train at a frequency f2 to charge said deflection yoke; and

means operative at the end of each pulse for permitting said yoke to discharge.

9. A circuit of the type described in claim 8 wherein said yoke discharge means includes:

a rectifying element; and

means for biasing said rectifying element to be substantially cutoff when pulses are being applied to said yoke and to conduct to provide a discharge path for said yoke when pulses are not being applied thereto.

10. A circuit of the type described in claim 9 wherein said rectifying element is a diode, the response time of which is such that it does not immediately become cut off when pulses are being applied to said yoke, whereby the charge and discharge transitions of said yoke are effectively smoothed.

11. A circuit of the type described in claim 10 wherein said first coupling means includes means for blocking the passage therethrough in either direction of a signal at a frequency f2; and

wherein said second coupling means includes means for blocking the passage therethrough in either direction of a signal at a frequency f1.

12. A circuit of the type described in claim 11 wherein said deflection yoke has an input terminal; and

wherein said first and said second coupling means both apply their respective input signals to said input terminal, whereby the inputs from said first and second coupling means are summed in said deflection yoke.

13. A circuit of the type described in claim 8 wherein said pulse train applying means includes a transformer the secondary of which is coupled to said yoke; and

means for applying a pulse train a frequency f2 to the primary of said transformer.

References Cited UNITED STATES PATENTS 3,204,145 8/1965 Schneider 31527 3,210,599 10/1965 Ward 3l5-27 3,281,822 10/1966 Evans 340324.1

RODNEY D. BENNETT, JR., Primary Examiner. C. L. WHITHAM, Assistant Examiner.

US. Cl. X.R.

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