the input of a charge pump 70 the output of charge pump 70 is coupled to filter circuits 100 and 200 in the manner shown in fig2 charge pump 70 causes a current i u to be pumped into a selected one of filters 100 and 200 when the error signal generated by phase detector 30 indicates that the f r signal at input 30a exhibits a frequency greater than the frequency of the signal applied at 30b the signal applied to phase detector input 30b is described in more detail subsequently charge pump 70 draws a current i d from a selected one of filters 100 and 200 when the error signal generated by phase detector 30 indicates that the frequency of the signal applied to input 30b is greater than the frequency of f r signal applied to input 30a the value of the i u current is dependent upon the extent which the frequency of the f r signal at input 30a exceeds the frequency of the signal at input 30b the i d value is dependent upon the extent which the frequency of the signal at input 30b exceeds the frequency of the f r signal at input 30a at various points of time while the synthesizer of fig2 is in operation the output of charge pump 70 is switchably coupled to the respective inputs of low pass filter networks 100 and 200 in the manner shown in fig2 and described in more detail subsequently the output of filter networks 100 and 200 are respectively switchably coupled to the input of a voltage controlled oscillator vco 40 in the manner shown in fig2 a storage.