Abstract:
A horizontal deflection circuit generates a horizontal deflection current in a deflection winding, during a trace interval and during a retrace interval of a deflection cycle. A transistor is responsive to a vertical rate parabola signal and to a negative feedback signal for producing a vertical rate parabolic modulation voltage at a collector of the transistor. A negative feedback network generates the feedback signal that is indicative of a current flowing in the transistor for increasing an output impedance at the collector of the transistor. An inductor is coupled to the collector of the transistor for producing in the inductor a modulation current to provide for side pincushion distortion correction in an East-West modulator.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
         [0001]    This is a non-provisional application which claims the benefit of a provisional application serial No. 60/316,236, filed Aug. 31, 2001.  
         BACKGROUND OF THE INVENTION  
         [0002]    This invention relates to raster distortion correction correction for a deflection circuit.  
           [0003]    In a television receiver, the electron beams inside the television receiver picture tube are deflected by magnetic fields generated by sawtooth deflection currents flowing in horizontal and vertical deflection windings. The deflected electron beams scan a raster pattern on the picture tube phosphor screen. The raster pattern, without correction, may display various geometric distortions such as side or east-west pincushion distortion. The correction of inner raster distortion requires modulation of the S-shaping as a function of the distance along the vertical axis of the raster display, i.e. from top to the center and from the center to the bottom of the raster. This is achieved by modulation of the voltage across the S-capacitor. The modulated voltage appears also across the horizontal deflection winding and causes a modulation of the deflection current.  
           [0004]    To correct side pincushion distortion, the peak-to-peak horizontal deflection trace current in the horizontal deflection winding is modulated at a vertical rate in a parabolic manner. Such parabolic modulation may be accomplished in a horizontal deflection circuit that includes an East-West modulator. Examples of East-West modulators that provide both side pincushion distortion correction and inner raster distortion correction are described in, for example, U.S. Pat. No. 3,906,305, entitled, CIRCUIT ARRANGEMENT FOR GENERATING A SAWTOOTH DEFLECTION CURRENT THROUGH A LINE DEFLECTION COIL, in the name of Nillesen (the Nillesen Patent).  
           [0005]    An East-West modulator circuit, embodying an inventive feature, includes an amplifier for generating a voltage across a capacitor at a vertical rate in a parabolic manner. The capacitor voltage is coupled via an inductor to the deflection circuit for providing pincushion correction. The amplifier includes a transistor stage having a main current conducting path which varies the voltage across the capacitor.  
           [0006]    The current through the inductor and the transistor of the modulator could include an undesirable decaying oscillating component or ringing following a fast transition in the input voltage to the amplifier, resulting in a width ringing at the top of the picture. It may be desirable to dampen the ringing in the deflection current by dampening the ringing in the capacitor.  
           [0007]    In carrying out an inventive feature, a current feedback path is formed in the amplifier. Consequently, an output impedance of the amplifier at a junction terminal between the capacitor and the inductor sufficiently increases to dampen the ringing and reduce picture distortion.  
         SUMMARY OF THE INVENTION  
         [0008]    A deflection apparatus embodying an inventive feature includes a deflection winding and a source of a synchronizing input signal at a frequency related to a first deflection frequency. A switching semiconductor is responsive to the first input signal and is coupled to the deflection winding for generating a deflection current in the deflection winding. A retrace capacitance is coupled to the deflection winding to form a retrace resonant circuit with the deflection winding during a retrace interval of a is deflection cycle. A source of a modulation input signal at a frequency related to a second deflection frequency is provided. A modulation amplifier is responsive to the modulation input signal for producing, from a current produced in an output stage of the modulation amplifier, a modulation voltage having a frequency related to the second deflection frequency to module the deflection current in a manner to provide side raster distortion correction. The modulation amplifier stage includes both a current feedback path that increases an output impedance of the output stage and a voltage feedback path that decreases the output impedance. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]    The sole FIGURE illustrates a deflection circuit with pincushion correction, embodying an aspect of the invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0010]    The sole FIGURE illustrates a horizontal deflection circuit  10 , embodying an aspect of the invention. An operating voltage B+is developed at a terminal  21  of a supply winding  22   a . Winding  22   a  is coupled to a collector terminal  43  of a conventional horizontal output transistor  27 . A base electrode of transistor  27  is coupled to receive from a conventional horizontal and driver stage  61   a  drive signal  27   a  at a horizontal rate of 31,468 Hz with a corresponding period H.  
         [0011]    Coupled to terminal  43  is also a series arrangement of a horizontal deflection winding  30 , a first trace or S-capacitor  31  and a winding W 1  of an inner pin correction transformer T 2 . A ringing suppression network  130  is formed by a diode  130   a  and a diode  130   b , coupled in series between a terminal  31   c  of capacitor  31  and a terminal  130   g . Ringing suppression network  130  also includes a resistor  130   c  and a resistor  130   d , coupled in parallel with diodes  130   a  and  130   b , respectively. Ringing suppression network  130  further includes a capacitor  130   e  coupled between terminal  130   g  and terminal  31   a  of capacitor  31 .  
         [0012]    A second trace or S-capacitor  31   b  is coupled between a secondary winding W 2  of transformer T 2  and terminal  31   a,  at a junction of winding W 1 , capacitor  130   e  and capacitor  31 . Thus, capacitor  31   b  is coupled in series with a secondary winding W 2  of transformer T 2 . The pair of S-capacitors  31  and  31   b  and winding  30  are effectively coupled in series.  
         [0013]    A damper diode  32  is coupled in parallel with a retrace capacitor  33  and between a terminal  41  of winding W 2  that is remote from capacitor  31   b  and a reference or ground potential. A horizontal damper diode  28  is coupled in parallel with a horizontal retrace capacitor  29  and between terminals  41  and collector terminal  43  of transistor  27 . The switching operation of transistor  27  produces a deflection current iy in winding  30  having a trace portion and a resonant, retrace portion, in a well known manner.  
         [0014]    A conventional East-West integration inductor or choke  34  is coupled between terminal  41  and a terminal  34   a  of a conventional filter capacitor  35 . Choke  34  has an impedance that is substantially higher, for example, five times higher, than that of deflection winding  30  at a resonant retrace frequency of current iy.  
         [0015]    Terminal  34   a  also forms an output terminal of an East-West modulation amplifier  60 . Modulation amplifier  60  produces at output terminal  34   a  a vertical rate parabola voltage V 34 . Voltage V 34  produces a vertical rate East-West modulation current im in choke  34 .  
         [0016]    Retrace capacitor  29 , deflection winding  30 , retrace capacitor  33  and the effective inductance associated with windings W 1  and W 2  of transformer T 2  form a bridge configuration similar to that of an East-West modulator described in the Nillesen Patent. Current im provides both a side or East-West pincushion raster distortion correction and an inner raster distortion correction in a similar way to that described in the Nillesen Patent. Deflection current iy flows in S-capacitors  31  and  31   b.  The amplitude of current iy varies in accordance with the vertical rate modulation produced by current im. The result is that a smaller amount of S correction is provided at the top and bottom and a larger amount of S correction is provided at the vertical center of the display screen, not shown. A current i 31  is produced by current iy in S-capacitor  31 . The modulation of a voltage V 31  in S-capacitor  31  changes as the amplitude of current iy changes for providing inner pin errors correction.  
         [0017]    A turns ratio, 2:1, associated with transformer T 2 , between the winding turns of winding W 1  and those of winding W 2 , is chosen to ensure that damper diode  32  continues to conduct, throughout the first half of horizontal scan, under all load conditions. The turn ratio associated with transformer T 2  establishes the ratio of inner-pin correction to side-pin correction.  
         [0018]    Non-inverting modulation amplifier  60 , embodying an inventive feature, includes an operational amplifier  80 . An output terminal, pin  7 , of amplifier  80  is coupled via a protection resistor R 25  to the base of a driver transistor  62 . An emitter of transistor  62  is coupled to ground via a current feedback sense resistor R 26 . A collector terminal  34   a  of transistor  62  forming an output stage of amplifier  60  is coupled via a voltage feedback resistor R 24  to a non-inverting input terminal, pin  5 , of amplifier  80 . Resistor R 24  is coupled to a resistor R 23  via non-inverting input terminal, pin  5 , of amplifier  80  to form a voltage divider for establishing the closed loop voltage gain of amplifier  60 , in accordance with the ratio between the values of resistors R 24  and R 23 . A vertical rate parabola signal Vin is coupled to an inverting input terminal, pin  6 , of amplifier  80 .  
         [0019]    As a result of a voltage feedback path provided by voltage feedback resistor R 24 , an output impedance at collector teminal  34   a  of transistor  62  is lower than an input impedance at terminal  34   a  of the reactive network formed by capacitor  35  and choke  34  at the frequency of voltage Vin. Without voltage feedback resistor R 24 , the output impedance at terminal  34   a  would be determined by the high collector output impedance of transistor  62 .  
         [0020]    In carrying out an inventive feature, a current feedback resistor R 29  is coupled between inverting input terminal, pin  6 , of amplifier  80  and the emitter of transistor  62  to provide a current feedback current iR 29 , in accordance with an emitter voltage Ve of transistor  62 . Emitter voltage Ve of transistor  62  is equal to a product of an emitter current ie in transistor  62  and the value of current sense resistor R 26 .  
         [0021]    Assume, for explanation purpose that feedback resistor R 29  were absent from amplifier  60 . As a result, a voltage gain, A, with respect to pin  6  of amplifier  80  would have been equal to 1+R 24 /R 23 , where the symbols R 24  and R 23  represent the values of the corresponding resistors. Because of the negative voltage feedback, an output impedance or resistance, Ro, at collector terminal  34   a  of transistor  62  would have been too small to provide any significant ringing dampening. Therefore, the current flowing in choke  34  and in transistor  62  emitter could include a decaying oscillating or ringing component following a fast transition in input voltage Vin, resulting in a width ring at the top of the picture, not shown.  
         [0022]    In carrying out an inventive feature, the inclusion of current feedback resistor R 29  in amplifier  60 , advantageously, increases the output impedance or resistance, Ro, at terminal  34   a  to, for example, 5 ohms, as follows: Ro=R 26 *R 22 /(R 22 +R 29 ))*(1+R 24 /R 23 ). The symbols R 24 , R 23 , R 29 , R 22  and R 26  represent the values of the corresponding resistors. The voltage gain, A, =(R 29 /(R 29 +R 22 ))*(1+R 24 /R 23 ). Thus, advantageously, the output impedance at terminal  34   a  at the low frequency of voltage Vin increases to an extent sufficient for quickly attenuating the ringing to avoid picture distortion. In this way, transistor  62  acts as an active resistor.  
         [0023]    Assume, hypothetically, that instead of using of current feedback in transistor  62  via resistor R 29 , a discrete, passive collector resistor having a value equal to Ro were coupled between capacitor  35  and terminal  34   a , in a matter not shown. Such passive collector resistor, not shown, would also produce output impedance or resistance, Ro, at collector terminal  34   a  of transistor  62 . However, such passive collector resistor, disadvantageously, would have had to have a relatively large power dissipation capability.  
         [0024]    Advantageously, the use of the current feedback in transistor  62  via resistor R 29  eliminates the need for using the aforementioned passive collector resistor, not shown. Instead, power is dissipated in transistor  62 . Sufficient power dissipation capability is anyhow required from transistor  62  for performing its task as a modulation driver transistor, even in the absence of the need to suppress ringing. Thus, advantageously, the ringing suppression function of transistor  62  does not add any burden on the requirements from transistor  62 .  
         [0025]    In addition to the thermal advantage of the use of transistor  62  as an active resistor over using passive collector resistor, not shown, transistor  62  acting as an active resistor has a greater dynamic range. In the aforementioned passive collector resistor approach, the minimum value of the output voltage at terminal  34   a  cannot be lower than the value, Ic 1 *(Ro+R 26 )+Vce_sat. The symbol Ic 1  denotes the value of collector current Ic at the vertical center of trace and the symbol Vce_sat denotes the saturation voltage of transistor  62 . Advantageously, by using current feedback in amplifier  60  via resistor R 29 , the minimum value of the output voltage at terminal  34   a  need not exceed the value, Il 1 *R 26  +Vce_sat. Consequently, the dynamic voltage range of the modulation voltage at terminal  34   a  is increased by the amount Ic*Ro relative to that obtained by employing the passive collector resistor approach.