Lecture 7 DIGITAL FREQUENCY METER Principle:

Fig 2.14: DFM principle The signal waveform whose frequency is to be measured is converted rted into tngger pulses and applied continuously to one terminal of an AND gate. To the other terminal of the gate, a pulse of 1 sec is applied as shown in the Fig. The number of pulses counted at the output terminal during period of 1 seec indicates the frequency. The signal whose fre requency is to be measured is converted too trigger pulses which is nothing but train of pulsses with one pulse for each cycle of the signal. At the output terminal of AND gate, the number numb of pulses in a particular interval of time t are counted using an electronic counter. Since each e pulse represents the cycle of the unknown signal, the number of counts is a direct indicaation of the frequency of the signal which ch is unknown. Since electronic counter has a high signals

can

be

speed

of

operation, high hi

frequency

measured.

The block diagram of digital ital frequency meter is shown in figure2.14 below. The signal waveform whose frequency is i to be measured is first amplified. Then the amplified signal is applied to the schmitt triggerr which converts input signal into a square wave with fast rise and

fall times. This square wave ave is then differentiated dif and clipped.

Fig 2.14: Basic circuit for measurement of frequency showing gate control F/F. As a result, the output from m the schmitt trigger is the train of pulses for each cycle of the signal. The output pulses frrom the schmitt trigger are fed to a START TART/STOP gate. When this gate is enabled, the input nput pulses pass through this gate and are re fed directly to the electronic counter, which counts the number of pulses. When this gate is disabled, the counter stops counting the he incoming incomi pulses. The counter displays the number nu of pulses that have passed through it in thhe time interval between start and stop. If this t interval is known, the unknown frequency can be measured. The output of unknown unkno frequency is applied to the Schmi chmitt trigger which produces positive pulse at the output. These are counted pulses present at A of the t11