Frequency Counter

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

• 1) What is a counter?
• 2) Ripple counter
• 3) Asynchronous 3-bit Up Down counter
• 4) What is a synchronous counter?

You may refer all those to clear your concept regarding how a counter works. Lets come to our current post.

What is frequency Counter?

A frequency counter is a circuit that can measure and display the frequency of a signal.The most basic function of a frequency counter is the measurement of an unknown frequency. As we know, now a days modern frequency counters, however, offer much more than just frequency measurement.

Other related parameters such as the time period, which is the reciprocal of frequency, the time interval between two events and the totalize count, which is nothing but the cumulative count over a known period, are other functions that are available with present-day frequency counters. These instruments, offering a variety of measurement options, basically called as universal counters.

Block diagram

 Now from the above block diagram, we can see the output of an AND gate, drive in frequency counter . And see the input of AND gate, those are

• Input pulses with unknown frequency, f_x.
•  A SAMPLE pulse.

Sample Pulse controls the time through which the pulses are allowed to pass through the AND gate into the counter.

 Basically the frequency counter is usually a 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.

Working principal

Now question is how this circuit works? Sampling interval is the time interval when the SAMPLE pulse goes HIGH from t₁ to t₂. 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.

Output

Now another question is that how accurate the output of frequency counter? so if we try to find answer of that question then we should looks on the accuracy of a frequency counter. The accuracy of it, 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.

Firstly, a crystal controlled oscillator is used to generate a very accurate 100 kHz waveform.

Secondly, it 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.

Finally, 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.

Let Check How you learn counter? with simple quiz.