As name suggest a frequency counter is a special kind of digital circuit that can measure and display the frequency of a signal. In my previous post I discussed on
You may refer all those to clear your concept regarding how a counter works.
Now from the above block diagram, we can see the frequency counter is driven by the output of an AND gate. And see the input of AND gate, those are input pulses with unknown frequency, fx, and in anther pin of AND gate a SAMPLE pulse that controls the time through which the pulses are allowed to pass through the AND gate into the counter.
Basically the frequency counter is usually made up of cascaded BCD counters. Now for displaying the measured output we can use the decoder/display unit converts the BCD outputs into a decimal display for easy monitoring.
Now question is how this circuit works? See when the SAMPLE pulse goes HIGH from t1 to t2; this is called the sampling interval. And in between this sampling interval the unknown frequency pulses will pass through the AND gate and will be counted by the counter. And see from above diagram after t2 the AND gate output becomes LOW and the counter stops counting. So in this way the counter counts the number of pulses that occur during the sampling interval.
Now another question is come that how accurate the output of frequency counter? The accuracy of a frequency counter is totally depends on the duration of the sampling interval, which must be very accurately controlled. See from above circuit how we control the sampling output. A crystal controlled oscillator is used to generate a very accurate 100 kHz waveform, which is shaped into square pulses and fed to a series of decade counters that are being used to successively divide this 100 kHz frequency by 10. The frequencies at the output of each decade counter are as accurate as the crystal frequency. We usually use Johnson counters as a decade counter. The switch is used to select one of the decade counter output frequencies to be fed to the clock input of a single flip-flop to be divided by 2.