TW344047B - Computer motherboard frequency generator with reduced electromagnetic interference - Google Patents

Abstract

A computer motherboard frequency generator with reduced electric magnetic interference (EMI), which comprises an input frequency eliminator M, a phase frequency detector, a control circuit, a charge pump and loop filter, an output frequency eliminator N, a voltage control oscillator, an EMI control circuit; in which a memory (ROM1) stores the number of input frequency eliminators N; three sets of memory codes (ROM-L, ROM-C, ROM-U) are installed for separately storing the values of three sets of lower, central and upper frequencies; the method used being : after the exterior has been set and after a short lock time, a reference signal f through the input frequency eliminator M becomes a frequency f1 (f1=fr/M); an output signal f0 of the voltage control oscillator through an output frequency eliminator N becomes a frequency f2 (f2=f0/N); f1 and f2 are jointly inputted into the phase frequency detector, and the phase difference obtained therefrom is fed to the control circuit thereby maintaining the output frequency fluctuation of the voltage control oscillator at a constant, then transmitting the output frequency to the charge pump and loop filter where the charge pump charges/discharges to the loop filter; the output voltage of the loop is used to control the output frequency f0 signal of the voltage control oscillator; in which the EMI control circuit is used to control the three sets of memory codes (ROM-L, ROM-C, ROM-U) such that the values contained in the three sets of memory codes alternately and variably output to the output frequency eliminator; in which the alternating relationship among the frequencies are shown in the following: f1=(M/N1)Xfr; f0=(M/N2)Xf4; fu=(M/N3)Xfr in which N1>N2>N3; f1<f0<fu.