Circuit for converting a frequency of an input signal so a signal having a digital value corresponding to the frequency

A measuring circuit for converting frequencies of a plurality of pulse train input signals having rising and trailing edges into a signal having digital values in correspondence with each of said frequencies, said measuring circuit also selectively displaying a measurement value on a display and comprising:

• clock pulse generating means for generating clock pulses having a frequency higher than the frequencies of said plurality of input signals;
• a plurality of edge detecting means, connected to said clock pulse generating means, for detecting either the rising edges or the trailing edges of said input signals after sampling said input signals on the basis of said clock pulses and for outputting a respective plurality of edge detection signals, which edge detection signals represent an existence/nonexistence of the edges of said input signals respectively, the edge detection signals being synchronized with said clock pulses to yield a plurality of synchronous signals;
• conversion control means, connected to said plurality of edge detecting means and said clock pulse generating means, for sequentially outputting said plurality of input signals synchronous with said clock pulses, and for sequentially generating and outputting a selection signal for said plurality of input signals, the generating and the output of said selection signal being performed on the basis of said clock pulses at every predetermined cycle, the output timing of said plurality of input signals output from said conversion control means being controlled on the basis of a time sharing process;
• selecting means, connected to said conversion control means, for receiving an input comprising said plurality of synchronous signals, and for selecting and outputting one of said plurality of synchronous signals, on the basis of said selection signal;
• and
• a first digital filter, connected in circuit to said selecting means to receive the synchronous signal selected by said selecting means, and connected in circuit to said clock pulse generating means to receive said clock pulses, for outputting a signal having digital values that correspond to the frequency of one of said synchronous signals selected by said selecting means, said outputting of the signal having the digital values being performed on the basis of a sampling synchronous with a cycle of said clock pulses.