The Principle of Operation of a Digital Frequency Meter

A frequency meter/counter is a digital instrument that can measure and display the frequency of any periodic waveform. Its operation is based on the principle of gating the unknown input signal into the counter for a predetermined time. For instance, if the unknown input signal were gated into the counter for exactly 1 second, the number of counts allowed into the counter would be precisely the frequency of the input signal. The term gated used here comes from the fact that an AND or an OR gate is used for handling the unknown input signal into the counter to be accumulated.

One of the simple methods used to construct a frequency counter is shown in the diagram above. It consists of a counter with its associated display/decoder circuitry, clock oscillator, a divider and an AND gate. The counter is usually made up of cascaded Binary Coded Decimal (BCD) counters and the display/decoder unit converts the BCD outputs into a decimal display for easy monitoring.

The Operation of the Frequency Meter/Counter

A GATE ENABLE signal of unknown time period is generated with a clock oscillator and a divider circuit and is then applied to one leg of an AND gate. The unknown signal is applied to the other leg of the AND gate and acts as the clock for the counter. The counter advances one count for each transition of the unknown signal, and at the end of the known time interval, the contents of the counter will be equal to the number of periods of the unknown input signal that have occurred during the time interval, t. For example, if the gate signal is of a time of exactly 1 second and the unknown input signal is a 600-Hz square wave; at the end of second the counter will count up to 600, which is exactly the frequency of the unknown input signal.

Let’s consider a simple example to demonstrate the operation of the frequency counter:

The waveform in Fig 1.1 above shows a clear pulse is applied to the counter at t₀ to set the counter at zero. Prior t₁, the GATE ENABLE signal is LOW, and therefore, the output of the AND get will be LOW and the counter will not be counting. The GATE ENABLE goes HIGH from t₁ to t₂ and during this time interval t (t₂–t₁), the unknown input signal pulses will pass through the AND gate and will be counted by the counter. After t₂, the AND gate output will be gain LOW because the GATE ENABLE signal is LOW and the counter will stop counting. Hence, the counter will have counted the number of pulses that occurred during the time interval, t of the GATE ENABLE SIGNAL, and the resulting contents of the counter are a direct measure of the frequency of the input signal. The accuracy of the measurement depends almost entirely on the time interval of the GATE ENABLE signal, which needs to be controlled very accurately.

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