Digital Frequency Meter

Every signal has a frequency whether it’s ac (50 Hz in India) or DC (0Hz). For appliances working on alternating current, the frequency must 50 Hz, if the frequency falls below 48 Hz then the voltage across the appliance increases. Thus, the appliance may get damage due to this overvoltage. For the safe working of appliances the frequency is needed to be measure.

The basic digital frequency meter needs the following components:-

  1. Amplifier
  2. Schmitt trigger
  3. AND gate
  4. A counter
  5. Crystal oscillator
  6. Time base selector
  7. A flip-flop

Functions of each component: –

As shown in above fig. various components and their functions are as mentioned below:

  • Amplifier:-

An amplifier is an electronic device that increases the voltagecurrent, or power of a signal.The most frequently-used device for power amplification is the bipolar transistor. This circuit usually amplifies the input signal applied to the input terminal to the amplifier. In this meter, this is an essential circuit which amplifies the magnitude of smaller input signals for measuring the frequency.

  • Schmitt Trigger:-

This circuit basically converts any alternating signal into a Square wave by use of operational amplifier.

In electronics, a Schmitt trigger is a comparator circuit with hysteresis implemented by applying positive feedback to the noninverting input of a comparator or differential amplifier. It is an active circuit which converts an analog input signal to a digital output signal.

  • AND gate:-

This gate basically provides an output when two inputs are present. In this meter, the input signals are from a Schmitt trigger connected on input signal side and a flip-flop connected to time base selector circuit.

The AND gate is a basic digital logic gate that implements logical conjunction – it behaves according to the truth table to the right. A HIGH output (1) results only if both the inputs to the AND gate are HIGH (1). If neither or only one input to the AND gate is HIGH, a LOW output results.

  • Counter:-

Counters are normally a synchronous type in which the decimal count is in ascending order starting from zero. In this meter, the output of a AND gate is given to counter to count the number of pulses.

A counter circuit is usually constructed of a number of flip-flops connected in cascade. Counters are a very widely used component in digital circuits, and are manufactured as separate integrated circuits and also incorporated as parts of larger integrated circuits.

  • Crystal oscillator: –

This circuit basically provides a sinusoidal output by using only a dc power supply. In this meter, the crystal oscillator connected is of 1MHz which produces a sine wave.

crystal oscillator is an electronic oscillator circuit that uses the mechanical resonance of a vibrating crystal of material to create an electrical signal with a precise frequency. This frequency is commonly used to keep track of time, as in quartz wristwatches, to provide a stable clock signal for digital integrated circuits, and to stabilize frequencies for radio transmitters and receivers. The most common type of piezoelectric resonator used is the quartz crystal, so oscillator circuits incorporating them became known as crystal oscillators.

  • Time base selector:-

It is a circuit which can change the time period of signals for the reference purpose. The time base consist of a fixed frequency crystal oscillator, called a clock oscillator, which has to be very accurate. In order to ensure its accuracy, the crystal is enclosed in a constant temperature oven. The output of this constant frequency oscillator is fed to a Schmitt trigger, which converts the input sine wave to an output consisting of a train of pulses at a rate equal to the frequency of the clock oscillator. The train of pulses then passes through a series of frequency divider decade assemblies connected in cascade. Each decade divider consists of a decade counter and divides the frequency by ten. Outputs are taken from each decade frequency divider by means of a selector switch; any output may be selected.

  • Flip-Flop: –

This circuit produces output depending upon the input given. It has four states depending upon which flip-flop is used.

flip-flop is a device very much like a latch in that it is a bistable multivibrator, having two states and a feedback path that allows it to store a bit of information. The difference between a latch and a flip-flop is that a latch is asynchronous, and the outputs can change as soon as the inputs do (or at least after a small propagation delay). A flip-flop, on the other hand, is edge-triggered and only changes state when a control signal goes from high to low or low to high. This distinction is relatively recent and is not formal, with many authorities still referring to flip-flops as latches and vice versa, but it is a helpful distinction to make for the sake of clarity.

Operation: –

  1. When we apply an input signal to the meter, the amplifier present in it starts amplifying the signal. The amplification is done in order to strengthen the weak magnitude of input signal.
  2. The amplified input is now given to Schmitt trigger which then converts the input signal into square wave.
  3. The square waves are then separated and clipped to obtain a train of pulses.
  4. Similarly, the oscillator produces a sine wave which is then converted into square wave which produces a square wave and is applied to the flip-flop.
  5. Now, the first pulse activates the gate control FF connected after the Schmitt trigger which enables the AND gate.
  6. Now, the train of pulses is then applied to AND gate due to which AND gate produces an Output.
  7. This Output is then given to the decimal counter which counts the first decimal number.
  8. Now when the second pulse is given to the Gate Flip-flop which removes the AND gate to “turn ON”.
  9. Thus, when the third pulse arrives at the flip-flop the AND gate is then enabled uptil then the counter shows the previous counted decimal.
  10. The decimal counter and display unit shows the output corresponding to the train of pulses received for a precise time interval.

Thus, the counter output shows only the frequency of signal.

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