Digital Frequency Meter – Block Diagram & Working

Functional Block diagram of Digital Frequency Meter

Fig. 1 Functional Block diagram of Digital Frequency Meter

Working principle of Digital frequency meter

1. The unknown frequency signal is fed to the Schmitt trigger. The signal may be amplified before being applied to Schmitt trigger.

2. In a Schmitt trigger, the signal is converted into a square wave with very fast rise and fall times, then differentiated and clipped.

3. As a result, the output from a Schmitt trigger is a train of pulses, one pulse, for each cycle of the signal.

4. The output pulses from the Schmitt trigger are fed to a start/stop gate when this gate opens (start), the input pulses pass through this gate and are fed to an electronic counter which starts registering the input pulses.

5. When the gate is closed (stop), the input of pulses to counter ceases and it stops counting.

6. The counter displays the number of pulses that have passed through it in the time interval between the start and stop. If this interval is known, the pulse rate and hence the frequency of the input signal can be known. The frequency (f) of an unknown signal is given by

f = N / t


f = frequency of an unknown signal

N = number of counts displayed by the counter

t = time interval between the start and stop of the gate

The range of modern digital frequency meter is between the range from 104 to 109 Hz.

Applications of Digital frequency meter

  1. It is used for testing radio equipment.
  2. It is used for measuring temperature, pressure, and other physical values.
  3. It is used for measuring vibration, strain

Q. The unknown input signal of 2 V square wave is 3.5 kHz. Determine the display indication if the gate enable time is (a) 0.1 seconds (b) 1 second and (c) 10 seconds.


Frequency of the signal, f = 3.5 kHz = 3500 Hz

Assuming 5 digit display

(a) t = 0.1 sec

f = N/t

N = ft = 3500 x 0.1 = 350

display = 00350

(b) t = 1 sec

f = N/t

N = ft = 3500 x 1 = 3500

display = 03500

(c) t = 10 sec

f = N/t

N = ft = 3500 x 10 = 35000

display = 35000

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