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Eliminating Cross Talk in CRT Deflection Yokes

IP.com Disclosure Number: IPCOM000087185D
Original Publication Date: 1976-Dec-01
Included in the Prior Art Database: 2005-Mar-03
Document File: 3 page(s) / 36K

Publishing Venue

IBM

Related People

Mortelmans, J: AUTHOR

Abstract

This design method eliminates cross talk due to resonating circuitry in cathode-ray tube (CRT) deflecting yokes.

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Eliminating Cross Talk in CRT Deflection Yokes

This design method eliminates cross talk due to resonating circuitry in cathode-ray tube (CRT) deflecting yokes.

Cross talk in deflection yokes is caused by the interwinding capacitance (between the horizontal and vertical) and parallel combinations of the different subsections of the vertical winding forming resonant circuits which are excited by the horizontal flyback pulses across the horizontal winding. Normally, there are several resonance peaks in the frequency response of the cross-talk current; however, it is the first one which is most responsible for creating raster distortion in CRT displays.

The flyback pulses are truncated cosine waveforms, as shown in Fig. 1a. When the horizontal period is chosen as T and the width of the pulse as T/A, the expression of the waveform is: f(t) = cos A Pi t/T -T/2A </- t </- T/2A

f(t) = 0 -T/2 </- t </- -T/2A, T/2A </- t </- T/2

f(t) = f(t+kT) = f(t-kT) k integer.

The Fourier series coefficients of this function are: b0 = 2/Pi A

bB = 4A over Pi(A/2/-4B/2/) cos B Pi/A

Where: B is an integer; bB = 1/A when B = A/2 and A is an integer.

Fig. 1b shows the Fourier transform of one truncated cosine pulse. The Fourier series coefficients are equal to twice the value shown in the plot, for k not = 0, because the transform is symmetric to f = 0 (that is, both positive and negative frequencies are included). However, for k = 0, it shows the correct value.

The zero crossings of the spectrum of the flyback pulse occur at harmonic f0 = A(k+0.5) (k is an integer and indexes the zero crossings). When A is an even integer, f0 is a harmonic; otherwise, it indicates the location of the zero crossing between two harmonics. This design method will aim at locating the cross talk resonance frequency at one of the spectral zero crossings. It is assumed here that for a given display application, the horizontal frequency (fh = 1/T) and the flyback pulse width (T/A) are known. The cross-talk resonance frequency was experimentally determined to be fct = 1 over 2 Pi x the Square Root of .55 Chv Lvp +/- 4% where Chv is the interwinding capacitance, and Lvp is the inductance of the vertical section between the center tap and the shorted terminals.

Typically, the vertical section consists of two su...