What is Frequency Counter : Circuit Diagram & Its Working

In digital electronics, counters are used to count the no.of pulses or events that have occurred. Counters store the data and made up of a group of flip-flops with an applied clock signal. Counters are capable of measuring frequency and time along with the counting process. These can increment memory addresses according to the application. Counters are divided into two types they are synchronous counters and asynchronous counters. The ‘mod’ of the counter indicates the no.of states should be applied before counting the pulses. These are used in various digital applications such as analog to digital converters, digital clocks, frequency dividers, timer circuits, and many more. This article is all about the frequency counter.

What is the Frequency Counter?

Definition: The test instruments which are associated with a wide range of radio frequencies that the frequency and time of digital signals are called frequency counters. These are capable of measuring the frequency and time of repeated digital signals accurately. These are also known as frequency meters, used to measure the frequency and time of square wave and input pulses. These are used various applications with RF range. These counters use Prescaler to reduce the frequency and operates the digital circuit. The frequency of the digital or analog signals is displayed on its display in HZ.

Frequency Counter
Frequency Counter

When the no.of pulses or events occurred in a specific period of time, the counter counts the pulses and transfers it to the frequency counter to display the frequency range of pulses and counter is set to zero. It is very easy to use and measure the frequency, and displays in digital form. These are available at affordable rates with more accuracy.

Block Diagram

The frequency counter block diagram contains input signal, input conditioning, and threshold, AND gate, counter or latch, accurate timebase or clock, decade dividers, flip-flop, and display.

Frequency Counter Block Diagram
Frequency Counter Block Diagram


When the input signal with high input impedance and low output impedance is applied to this counter, then it will be fed to the amplifier to convert the signal into a square wave or rectangular wave for processing within the digital circuit. The input signal is buffered and amplified by using the input conditions and thresholds. In this stage, Schmitt trigger is used to control the counting additional pulses occurred due to noise at the edges. To reduce the counting additional pulses, the trigger level and sensitivity of the counter can be controlled.

Clock (Accurate Time-base)

Clock or accurate time-base is necessary to produce various timing signals at precise time intervals. It uses a crystal oscillator with high quality for controlled and accurate timing signals. The clock is applied to decade dividers.

Decade Dividers and Flip-Flop

Pulses generated from the incoming signal and clock signal are fed to the decade dividers to divide the clock signal and the output is given to the flip-flop to produce enabling pulse for the main AND gate.


The accurate enabling pulse from the flip-flop and the train of pulses from the input signal is applied to the gate ( AND gate ) to produce a series of pulses at a precise time interval. If the input signal/incoming signal is at 1 MHZ and for the 1-second gate should be opened, then 1 million pulses are produced as a resultant output signal.

Counter or Latch

The output of the gate is fed to the counter to count the no.of pulses that occurred from the input signal. The latch is used to hold the output signal while displaying the figures, meanwhile, the counter counts the pulses. It will have 10 stages to count and hold the pulses.


The output of the counter and the latch are given to the display to provide the output in a readable format. The frequency of the output signal is displayed. The most commonly used displays are LCD or LED. Since there will be one digit for each decade counter and the related information is displayed on the display.

Frequency Counter Circuit Diagram

The circuit diagram of this can be done using two timers, counters, 8051 microcontrollers, potential resistors, square wave generator, and LCD display. The basic circuit diagram is shown below.

Circuit Diagram Using Timers
Circuit Diagram Using Timers

The frequency counter uses IC 555 timer to provide clock signals at a precise time interval of one second. Arduino UNO is used as a square wave generator. An IC 555 timer and square wave generator can be configured as an astable multivibrator. The 16×2 LCD display is used to display the frequency of the output signal in Hertz.

The circuit of this can be done by using IC 555 timer and timer/counter of 8051 microcontrollers. To generate the oscillating signals with a duty cycle (99%) with the highest time period of the output signal, the IC 555 timer is used. The threshold and discharge resistors can be adjusted to get a desired value of the duty cycle. The formula for the duty cycle is D= (R1+R2)/(R1+2R2).

The timer/counter of 8051 microcontrollers is used to generate the frequency of the pulse in Hertz. Since 8051 has two timers acts as timer 0 and timer 1 and operated in mode 0 and mode 1. Timer 0 is used to produce a time delay. The pulses out from the square wave generator are counted by using timer 1.

The circuit design of the frequency counter using IC 555 timer is shown below.

Frequency Counter Using IC 555 Timer
Frequency Counter Using IC 555 Timer

Frequency Counter Circuit Operating Principle

The pulses generated from the square wave generator are fed to the counter/timer of 8051. It is operated in two modes to generate time delay and count the pulses. The counter/timer of 8051 counts the no.of pulses from the input signal at a time interval. The output from the counter is given to the 16×2 LCD display to display the frequency of the signal (no.of cycles/second) in Hz at a particular time interval. This is the operating principle of the frequency counter.

Frequency Counter Working

The working of the frequency counter can be explained from the above circuit diagram. The pulse generated from the square wave generator (Arduino UNO) is given to the pin 3.5 (port 3) of 8051 microcontrollers. Pin 3.5 of 8051 acts as timer 1 and configured as a counter. TCON TR1 bit can be set to HIGH and LOW to count the pulses. The final count is stored in TH1 and TL1 registers (timer 1). The frequency of the pulse can be calculated by using the formula,

F = ( TH1 X 256 ) + TL1

To convert the values of the pulse in hertz, the resultant value is multiplied by 10 i.e., frequency in cycles per second. After some calculations inside the frequency counter, the frequency of the pulse is displayed on 16×2 LCD.

Types of Frequency Counter

The frequency of the pulse can be measured by using two types of frequency counters. They are,

  • Direct counting frequency counter
  • Reciprocal frequency counter.

Direct Counting Frequency Counter

This is one of the simplest methods of measuring the frequency of an input pulse. After counting the no.of cycles of the input pulse per second, the frequency can be calculated by using a simple counter circuit. This conventional method is limited to measure low-frequency resolution. To get the highest resolution, the gate time can be expanded. For example, to measure resolution at 1MHZ, then 1000 seconds time period is needed to measure at one time.

Reciprocal Frequency Counter

This method is used to overcome the disadvantages of the direct counting method. It measures the time period of input pulse instead of calculating no.of cycles per second. The frequency of the pulse can be calculated by using F = 1/T. The final frequency resolution depends on temporal resolution and independent of the input frequency. It can measure the low frequency at the highest resolution very quickly and reduces the noise by adjusting the trigger level. It measures the time period of input pulse ( contains several cycles ) and maintains sufficient time resolution. This can be carried out at a low cost.

The other types of frequency counters are

  • The bench frequency counter is used for electronics test equipment
  • PXI frequency counter displays frequency in a PXI format and used for test and control systems.
  • Handheld frequency counter
  • Frequency counter using a digital multimeter
  • Panel meter


The advantages of frequency counter are

  • It measures the frequency of the pulse generated from the square wave generator at a precise time interval.
  • These are widely used to measure frequency within the RF range
  • These counters provide accurate frequency values very quickly and easily.
  • It is cost-effective depending on the application.
  • Ensures that all the frequencies are transmitted within the specified bands.


The applications of frequency counter are

  • Used to determine the frequency of the pulse obtained from the square wave generator.
  • Used to measure the frequency of the pulse very accurately
  • Measures the frequency of the incoming signal at the transmitter and receiver on a line
  • Used in data transmissions because of the clock pulse.
  • Frequency of an oscillator can be measured
  • Used in RF Range
  • Detects the frequency of a high power data transmissions


1). What is the unit of frequency?

Frequency of the signal is measured in Hertz (HZ)

2). What is the use of a frequency counter?

These are used to measure the accurate frequency of a signal generated from a square wave generator or an oscillator.

3). What type of counters are used to measure high frequencies?

Synchronous and asynchronous counters are used to measure high frequencies.

4). What do you mean by the mod counter?

Mod counter or modulus counter is defined as the no.of states that counter counts the pulse in sequence by applying a clock signal.

5). What are the two methods of frequency counter?

The methods are Direct counting and Reciprocal

Thus, this is all about the definition, block diagram, circuit diagram, circuit design, operating principle, working, types, advantages and applications of the frequency counter. Here is a question for you, what are the disadvantages of a frequency counter?

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