Frequency counter in digital electronics
The frequency of a pulse signal can be measured and displayed using a counter. The counter is driven by the output of an AND gate. The inputs to the AND gate are:
i. The pulse signal whose frequency is to be measured.
ii. A sample pulse that is HIGH between time t1 and t2.
The output of the AND gate will be held LOW except during the time interval (t1 to t2) called the sampling interval. During this time, the pulse signal of the unknown frequency will appear at the output of the AND gate. It will be counted by the Counter. After t2, the output of the AND gate stays LOW, and hence the counter stops counting. Thus, the counter counts the number of pulses that occur during the sampling interval. It is a direct measure of the frequency of the pulse waveform.
The accuracy of this method depends on the duration of the sampling interval which must be very accurately controlled. A commonly used method for obtaining very accurate sample pulses is shown below figure:
A crystal-controlled oscillator generates a very accurate waveform of frequency equal to 100KHz. This waveform is shaped into square pulses and fed to a series of decade counters which are being used to successfully divide this 100KHz frequency by 10. This switch is used to select one of the decade counter outputs which will be fed to the CLK input of the flip-flop to be divided by 2.