Principle of operation:

• The signal waveform is converted to trigger pulses and applied continuously to an AND gate, as shown in Figure 1.
• A pulse of 1 sec is applied to the other terminal, and the number of pulses counted during this period indicates the frequency.
• The signal whose frequency is to be measured is converted into a train of pulses, one pulse for each cycle of the signal.
• The number of pulses occurring in a definite interval of time is then counted by an electronic counter.
• Since each pulse represents the cycle of the unknown signal, the number of counts is a direct indication of the frequency of the signal (unknown).
• Since electronic counters have a high speed of operation, high frequency signals can be measured.

Figure 1 

Block Diagram:

Block diagram of Digital Frequency meter is shown below. (Figure 2). The assembly consisting of two gates and two flip-flops is knows as gate control flip- flop.

Figure 2

Working:

• The input signal is amplified and converted to a square wave by a Schmitt trigger circuit.
• In the diagram, the square wave is differentiated and clipped to produce a train of pulses, each pulse separated by the period of the input signal.
• The time base selector output is obtained from an oscillator and is similarly converted into positive pulses.
• The first pulse activates the gate control F/F. This gate control F/F provides an enable signal to the AND gate. The trigger pulses of the input signal are allowed to pass through the gate for a selected time period and counted.
• The second pulse from the decade frequency divider changes the state of the control F/F and removes the enable signal from the AND gate, thereby closing it.
• The decimal counter and display unit output corresponds to the number of input pulses received during a precise time interval; hence the counter display corresponds to the frequency.