Basic Circuit of Digital Frequency Meter

Basic Circuit of Digital Frequency Meter Points : Basic Circuit of Digital Frequency Meter The signal may be amplified before being applied to Schmitt trigger. In a Schmitt trigger the signal is converted into a square wave with 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 start stop gate. When this gate opens (start), the input pulse. pass through this gate and arc ted to an electronic counter, which starts registering the input pulses. When the gate is closed (stop), the input of pulses to counter ceases and stops counting.

The counter displays the number of pulses that have passed through it in the time interval between start and stop. It. this interval is known, the pulse rate and hence the frequency of the input signal can be known. Suppose “f’ is the frequency of unknown signal, “N” the number of counts displayed by counter and “t” is the time interval between start and stop of gate. Thus the frequency of unknown signal f=N/t.

It is abundantly clear that in order to know the value of frequency of input signal, the time interval between start and stop of gate must be accurately known. This time interval is known as “time base” and can be determined by an other circuit. This circuit consists of clock oscillator Schmitt trigger and a selector switch. Further the operation of this circuit is out of the scope of this topic.

A block diagram of a simple digital counter having facilities for frequency measurement, multi-period measurement and time measurement is shown. The frequency measurement is achieved by counting the number of cycles of the input signal which occur during a fixed time interval. If the fixed time is I second then the count recorded at the end of the time period will be the input frequency in Hz. By selecting other time intervals, either decade multiples of sub multiples of 1 second, various ranges and resolutions may be catered for.