Method and apparatus for reducing electromagnetic interference radiated

Abstract

Electromagnetic interference ('EMI') generated by a flat panel video display system is reduced by periodically phase/modulating the panel clock. This spreads EMI energy associated with each panel clock harmonic by a frequency amount .DELTA.f proportional to the rate of phase change in the panel clock signal. EMI energy associated with each panel clock harmonic is reduced relative to a square-wave panel clock signal because the same energy is now spread over a group of frequencies centered about each harmonic. The phase of the panel clock is changed at a rate exceeding the bandwidth f.sub.m of a standard EMI measurement reference window. This disperses adjacent spectral energy sufficiently so the reference window measures but one, decreased, amplitude at a time. Phase-modulation may be achieved using a clock pulse dropping circuit that receives a square-wave input of frequency Nf.sub.c from a main oscillator, drops at least one clock pulse out of a stream of M clock pulses, and frequency divides the resultant waveform by N. The resultant panel clock will have two phases, wherein rate of phase change .DELTA.f is Nf.sub.c /2M. The spectral spacing .DELTA.f of sidebands around each harmonic in the EMI spectrum changes as a function of .DELTA.f. Measured EMI is reduced when .DELTA.f=Nf.sub.c /2M>f.sub.m.