Fractional-N divider, fractional-N phase locked loop and method of dividing a frequency f of an output signal by N, wherein N is a non-integer
A fractional-N divider for dividing a frequency f of an output signal by N, where N is a non-integer. The fractional-N divider includes an oscillator 209 adapted to provide K output signals I,I=I 1 , . . . , I K . Each output signal I has the same frequency f and period T. The output signals I are mutually phase shifted by T/K or a multiple of T/K. The fractional-N divider further comprises a multiplexer 211 adapted to select one signal from the K output signals I. The selected signal is phase shifted by M*T/K in relation to a previously selected signal, where M is an integer. Additionally, the fractional-N divider comprises a counter 206 adapted to receive said selected signal. The counter 206 is adapted to count a predetermined number X of periods of the selected signal, whereupon the counter 206 outputs a counter signal 230 . The counter 206 is connected to a control input of the multiplexer 211 . The multiplexer 211 is adapted to switch from a currently selected output signal to another selected output signal in response to said counter signal 230.
1 . A fractional-N divider for dividing a frequency f of an output signal by N, wherein N is a non-integer, comprising:
an oscillator ( 209 ) adapted to provide K output signals I,I=I 1 , . . . , I K , wherein each output signal I has the same frequency f and period T and the output signals I are mutually phase shifted by T/K or a multiple of T/K;
a multiplexer ( 211 ) adapted to select one signal from the K output signals I, the selected signal being phase shifted by M*T/K in relation to a previously selected signal, where M is an integer;
a counter ( 206 ) adapted to receive said selected signal and count a predetermined number X of periods of the selected signal, whereupon the counter ( 206 ) outputs a counter signal ( 230 ); and
wherein the counter ( 206 ) is connected to a control input of the multiplexer ( 211 ) and the multiplexer ( 211 ) is adapted to switch from a currently selected output signal to another selected output signal in response to said counter signal ( 230 ).
2 . A Fractional-N phase locked loop comprising the fractional-N divider according to claim 1 , wherein said oscillator is a voltage controlled oscillator ( 209 ), further comprising a phase detector ( 102 ) having an input connected to the counter ( 206 ) for receiving the counter signal ( 230 ) and an output connected to an input of the voltage controlled oscillator ( 209 ).
3 . A Method of dividing a frequency f of an output signal by N, wherein N is a non-integer, comprising the steps:
generating K output signals I, I=I 1 , . . . , I K , wherein each output signal I has the same frequency f and period T and the output signals I are mutually phase shifted by T/K or a multiple of T/K;
selecting one signal from the K output signals I, the selected signal being phase shifted by M*T/K in relation to a previously selected signal, where M is an integer;
counting a predetermined number X of periods of the selected signal, whereupon a counter signal ( 230 ) is output; and
switching from a currently selected output signal I to an output signal I phase shifted by M*T/K in response to said counter signal ( 230 ).