Start by identifying the signal you need to measure

Hello everyone. Frequency is the number of cycles of a repeating event per second, expressed in hertz (Hz). Electrical signals can be measured with a multimeter, oscilloscope or frequency counter; rotating motion can be measured using an encoder, tachometer or stroboscope. First establish what you need to measure: the supply frequency, a sensor's pulses, or the rotation of a motor shaft.

Two basic relationships are useful: f = N / Δt and f = 1 / T. Here, N is the cycle count, Δt is the counting interval, and T is the duration of one complete cycle. For example, 100 cycles in two seconds gives 50 Hz. The same signal has a period of 20 ms = 0.020 seconds. These examples assume a stable, periodic signal.

Practical measurement with a multimeter

On a multimeter with a frequency function, connect the black lead to COM and the red lead to the manufacturer's specified V/Ω/Hz input. Depending on the instrument, select Hz directly or use the Hz button in the AC voltage function. For voltage-based measurement, connect the probes in parallel across the signal; do not use the current input for this connection. Check the voltage limit, measurement category and probe ratings before connecting.

Every meter has its own frequency range, input sensitivity and trigger threshold. Low amplitude, noise or waveform distortion can produce incorrect or unstable readings. True RMS capability alone does not guarantee frequency accuracy or suitability for drive output measurements. Live measurements on mains and power circuits require qualified personnel and suitable equipment.

Frequency counters: count pulses or measure their period

With direct counting, the instrument counts edges of the same polarity during a known gate time. For example, 1,000 pulses in 0.1 seconds gives 10 kHz. The ideal counting increment is approximately 1 / gate time, so a longer gate provides finer resolution but a slower response to changes. The input's supported counting frequency must not be exceeded.

At low frequencies, a short gate captures very few pulses. Instead, the instrument can time one or several cycles against a precise clock and calculate frequency; this is known as reciprocal measurement. Both approaches depend on timebase accuracy and reliable triggering. The number of display digits is not the same as measurement accuracy.

Seeing one complete cycle on an oscilloscope

An oscilloscope shows the waveform as well as a numerical result. After establishing stable triggering, measure the period T between successive rising edges or zero crossings in the same direction. With a horizontal scale of 5 ms/division and four divisions per cycle, T = 20 ms and f = 50 Hz. Choose a probe ratio, voltage scale, sampling rate and bandwidth appropriate to the signal.

Do not connect the probe ground of an earth-referenced bench oscilloscope to a mains phase or motor drive output. These measurements require a differential probe with suitable voltage and category ratings or a suitably rated isolated-input instrument. Never disconnect the oscilloscope's protective earth to make a measurement.

Avoid doubling the result when counting zero crossings

A clean, zero-centred sine wave crosses zero twice per cycle. When counting crossings in both directions, use f = crossing count / (2 × time). If only rising crossings are counted, do not divide by two. Noise can generate extra crossings near the threshold, so appropriate filtering and hysteresis may be needed. Signals with DC offset or distortion require additional care when using this method.

Which frequency are you reading on a motor drive?

The output of a variable frequency drive consists of PWM pulses. Its fundamental motor frequency might be 40 Hz while the carrier frequency is several kilohertz. An ordinary meter may detect the switching frequency or noise-related transitions. To measure the fundamental, use a VFD measurement mode or low-pass filter specifically approved for that purpose by the instrument manufacturer.

Compare the measurement with the drive's actual output frequency display, rather than relying solely on the frequency reference. Ramps or active limits can make the output differ from the command. The motor's rated frequency is also different from the frequency currently applied. Furthermore, electrical Hz is not directly equal to rpm: pole count and induction motor slip must be considered, and actual shaft speed can be measured separately.

Calculating shaft speed from encoder pulses

If one channel of an incremental encoder produces P pulses per revolution, use n = 60 × f / P. For example, an encoder producing 1,000 pulses per revolution on channel A gives a shaft speed of 1,500 rpm when that channel measures 25,000 Hz. The encoder output type and voltage level must match the measuring input.

Encoder readout devices using all four edges of the A/B signals provide an x4 count, which differs from the pulse rate of a single channel. Distinguish pulses per revolution from processed counts per revolution. If a speed sensor detects each tooth on a gear, P is the number of teeth detected per revolution. With high-resolution encoders, the input frequency rises quickly at high shaft speeds.

Separating the fundamental and harmonics with FFT

When a signal contains several components, examine its frequency spectrum. An FFT reveals the frequency components of a sampled signal, helping distinguish the fundamental, harmonics and interference. The same approach applies to audio signals. The tallest spectral peak is not necessarily the fundamental you are looking for.

An insufficient sampling rate can make high-frequency components appear at lower frequencies; this is called aliasing. Consider band limiting and sampling together. The FFT's natural frequency-bin spacing is approximately Δf = 1 / record duration, while the selected window also affects peak shape. Extra display digits do not remove these limitations.

When are other methods useful?

A frequency-to-voltage converter produces an analogue voltage scaled to input frequency. For example, if a circuit is designed to map 0–1,000 Hz to 0–10 V, a 2.5 V output represents approximately 250 Hz. Scaling, offset, filtering and calibration affect the result; connecting a voltmeter alone is not enough.

A PLL can track a signal by maintaining a locked phase relationship. Determining frequency requires knowledge of the oscillator, divider ratios and reference; a PLL is not a universal frequency meter by itself. A heterodyne system mixes the signal with a known local oscillator to translate it to a more manageable intermediate frequency. These techniques are used in communication systems. With frequency-modulated signals, distinguish the carrier frequency from its variation over time.

A stroboscope uses adjustable light pulses to make a rotating part appear stationary. Similar images can also occur at multiples or submultiples of the rotation rate. Verify the result using a unique reference mark and different flash settings. A part that appears stationary is still rotating.

Choosing where to start

For a stable, single-frequency signal within the instrument's limits, a multimeter with a Hz function may be practical. Use an oscilloscope to inspect the waveform and triggering, a frequency counter for precise pulse measurements, and FFT or a spectrum analyser to separate components. Record the measuring point, load conditions, instrument mode and filter setting alongside the result.