Principal of digital frequency meter.
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Principle: The signal may be amplified before being applied to the Schmitt trigger. The Schmitt trigger converts the input signal into a square wave with fast rise and fall times, which is then differentiated and clipped. As a result the output from the 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 is enabled, the input pulses pass through this gate and are fed directly to the counter which counts the number of pulses. When gate is disabled the counter stops counting the incoming pulses. The counter displays the number of pulses that have passed through it in the time interval between start and stop. If this interval is known the pulse rate and hence the frequency of the input signal cab be known. If f is the frequency of unknown signal, N is the number of counts displayed by counter and t is the time interval between start and stop gate then, frequency of unknown signal is, f = N / t

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The basic block diagram of basic digital frequency meter (DFM) is shown in above figure. The signal whose frequency we have to be measured is first to be amplified through amplifier. The output of amplifier is applied to the Schmitt trigger. The Schmitt trigger is convert input signal into a square wave which has a fast rise and fall time. The square wave is then differentiated and clipped. Each pulse is proportional to each cycle of unknown signal. Now the output from Schmitt trigger is applied to a start and stop gate. The input pulses are allowed to pass through it, when the gate is open. The counter starts to count these pulses. The gate is closed the output pulses are not allowed to pass through the gate. The counter stops the counting. When the gate is open the number of pulse are counted by the counter. The interval between start and stop condition is the frequency of unknown signal which has to be measured.


F = N/t
Where,
F = Unknown frequency.
N = Number of counts.
t = Time interval between start and stop condition of the gate.
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