Q. Explain digital frequency meter.

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Explain principle of digital frequency meter and discuss utility of time base selector with suitable diagrams.

Sol. Digital Frequency Meter: The signal whose frequency is to be measured is converted into a train of pulses, one pulse one pulse for each cycle of the signal. Then the number of pulses appearing in a definite interval of time is counted by means of an electronic counter, since the pulses represent the cycles of unknown signal, the number appearing on the counter is a direct indication of frequency of the unknown signal. Since the electronic counters are extremely fast, the frequency of high frequency of signals may be known.

    Basic circuit: The block diagram of the basic circuit of a digital frequency meter is shown. The unknown frequency signal is fed to a Schmitt trigger.

    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 pulses pass through this gate and are fed to an electronic counter which starts registering the input pulse. 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 start and strop. If 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. Therefore frequency of unknown signal f = N/t.

Time base : In order to know the value of frequency of input signal, the time interval known as time base can be determined by the circuit given in. The time-base consists of a fixed frequency crystal oscillator. This oscillator, which is known as clock oscillator must be very accurate. In order to insure its accuracy, the crystal is enclosed in a constant temperature oven. Then output of this constant frequency oscillator is fed to a Schmitt trigger which converts the input to an output consisting of a train of pulses then passes through a series of frequency divider decade assemblies connected in cascade. Each decade divider consists of a decade counter divides the frequency by 10. Connections are taken from the output of each decade in the series chain and, by means of a selector switch any output may be selected.

  In the block diagram of the clock oscillator frequency is 1MHz or 10 Hz. Thus the output of Schmitt trigger is pulses per second. At the tap of the switch there are pulses per second and thus the time interval between two consecutive pulses is second or 1 µs.

At tap, the pulses having gone though decade divider 1, are reduced by a factor 10 and now there are 10⁶ pulses per second. Therefore the time interval between them is 10µs. Similarly, there are 10⁴ pulses per second at tap 10^-2  and the time interval is 100µs ; 10^-3 pulses at tap 10^-3 and the time interval is 1 ms; 100 pulses per second at tap  10^-4 and the time interval 10 ms, 10 pulses per second at tap10^-5  and time interval 100 ms; one pulse per second at tap 10^-6 and the time interval is 1 second.

 This time interval between the pulses is the time base and it can be selected by means of the selector switch.