Electronic signals are rarely perfect. Even when a waveform appears normal on an oscilloscope, it can contain additional frequency components that affect circuit performance. Harmonics and distortion are common examples. By using FFT analysis with an oscilloscope, engineers can examine these components and gain a better understanding of signal quality.

What Are Harmonics?

A harmonic is a frequency component that occurs at an integer multiple of a signal's fundamental frequency. For example, if the fundamental frequency of a signal is 1 kHz, its second harmonic appears at 2 kHz, the third at 3 kHz, and so on.

A perfectly sinusoidal signal contains only its fundamental frequency. When additional harmonic components are present, the waveform can become distorted.

These harmonic components may be caused by nonlinear components, amplifiers, power circuits, switching devices, or other parts of an electronic system.

How Oscilloscope FFT Reveals Harmonics

A conventional oscilloscope shows the waveform in the time domain. Engineers can observe whether the waveform appears distorted, but determining the exact frequency components can be difficult.

Using Oscilloscope FFT, the captured waveform can be converted into a frequency-domain display. The fundamental frequency normally appears as a prominent peak, while harmonics appear at higher multiples of that fundamental frequency.

This makes it easier to determine whether a signal contains unwanted frequency components.

For example, an amplifier may receive a 10 kHz sine-wave input. If the output contains significant components at 20 kHz and 30 kHz, the FFT display can reveal the second and third harmonics. Engineers can then investigate the cause of the distortion.

Understanding Signal Distortion

Distortion occurs when the output signal does not accurately reproduce the expected shape of the input signal. It can result from amplifier limitations, nonlinear components, clipping, incorrect biasing, or excessive signal levels.

Some distortion is easily visible in the time-domain waveform. However, smaller amounts may be easier to identify using FFT frequency domain analysis.

By comparing the amplitude of the fundamental frequency with the amplitudes of its harmonics, engineers can gain useful information about the severity and characteristics of the distortion.

Practical Testing Applications

FFT analysis with an oscilloscope can be used for a variety of electronic testing applications, including:

• Audio amplifier testing

• Power supply analysis

• Sensor signal evaluation

• Motor drive testing

• Communication circuit development

• Function generator verification

• Analog circuit troubleshooting

• Digital circuit noise investigation

For audio equipment, for example, FFT can help engineers identify unwanted harmonic components generated by an amplifier or other signal-processing circuit.

Setting Up an FFT Measurement

The measurement begins by connecting an appropriate probe to the circuit under test. Engineers should ensure that the probe is properly grounded and that the connection does not introduce unnecessary interference.

The oscilloscope's sampling rate and record length should be selected according to the frequency being measured. A suitable record length can improve frequency resolution and make closely spaced components easier to distinguish.

Window selection is also important because spectral leakage can affect the appearance of frequency peaks.

Comparing Input and Output Signals

One useful troubleshooting technique is to compare the FFT of an input signal with the FFT of the corresponding output signal.

If the input contains primarily one frequency but the output contains several additional peaks, those peaks may indicate distortion introduced by the circuit.

This approach can help engineers identify which stage of a system is responsible for unwanted harmonic generation.

Avoiding Misleading Results

Not every peak on an FFT display necessarily comes from the circuit being tested. Probe pickup, grounding problems, electromagnetic interference, and inappropriate acquisition settings can introduce misleading frequency components.

Engineers should therefore verify unexpected peaks by changing the measurement setup or testing the instrument with a known reference signal.

Harmonic distortion can provide valuable information about the behavior of electronic circuits. Oscilloscope FFT makes it easier to identify these frequency components and investigate their sources.

By combining time-domain measurements with FFT frequency domain analysis, engineers can examine both the shape of a signal and the frequencies contributing to it. This makes FFT analysis with an oscilloscope a practical technique for amplifier testing, power electronics, sensor evaluation, and general electronic troubleshooting.