In a digital frequency meter, the Schmitt trigger is used for

Digital frequency meter: It is a general purpose instrument that displays the frequency of a periodic electrical signal to an accuracy of three decimal places. It counts the number events occurring within the oscillations during a given interval of time.

Block Diagram:

Working principle:

• The unknown frequency signal is fed to the Schmitt trigger. This signal may be amplified before being applied to Schmitt trigger.
• In a Schmitt trigger, the signal is converted into a square wave with a very fast rise and fall times, then differentiated and clipped.
• As a result, the output from a Schmitt trigger is a train of pulses, one pulse, for each cycle of the signal.
• 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.
• When the gate is closed (stop), the input of pulses to counter ceases and it stops counting.
• The counter displays the number of pulses that have passed through it in the time interval between the start and stop. If this interval known, the pulse rate and hence the frequency of the input signal can be known.

The frequency of unknown signal is given by

f = N/t

where

N is the number of counts displayed by counter

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