Clock synthesizer for low emi applications

Abstract

The object of the present invention is to provide a clock synthesizer IC which can produce clock signals with much lower radiated EMI. The clock synthesizer IC comprises: a forward programmable counter for dividing the frequency of input clock signals by M; a feedback programmable counter for dividing the frequency of input clock signals by N; a phase-frequency detector for accepting one input signal from said forward programmable counter and another from said feedback programmable counter, outputting two signals whose levels are determined by phase difference of said two input signals; a loop filter accepting the output signal of said phase-frequency detector and outputting a signal with high-frequency component filtered out; a voltage-controlled oscillator accepting the output signal of said loop filter and outputting a signal as the input signal of said feedback programmable counter and as the output signal of the whole clock synthesizer whose frequency is determined by input voltage; a programmable logic array, for controlling the dividing-number M and N of said forward programmable counter and said feedback programmable counter respectively. An up/down counter for controlling the dividing-number N of said feedback programmable counter in cooperation with said programmable logic array by way of dividing the binary representation of said dividing-number D.sub.n D.sub.(n-1) . . . D.sub.1 D.sub.0 into two groups: D.sub.n . . . D.sub.k and D.sub.(k-1) . . . D.sub.0 and controlling them by said programmable logic array and said up/down counter respectively in which the time interval of transition of D.sub.n D.sub.(n-1) . . . D.sub.1 D.sub.0 is determined by said up/down counter.