Oscilloscope's FFT Analysis Shows Frequency Content

This picture shows an advanced feature of Oscium’s handheld oscilloscope: the built-in FFT function. Until modern digital oscilloscopes replaced their analog ancestors, it was difficult for engineers and electronics enthusiasts to analyze the frequency component of a signal. Today’s digital oscilloscopes have screen capture capabilities, Ethernet connectivity, but most importantly they have powerful microprocessors and high speed ADCs that capture and analyze signals – almost instantaneously. One of the complex math functions implemented with DSP and microprocessors is the Fast Fourier Transform, or just “FFT”. The setup for this picture uses a high gain audio amplifier and a microphone to create an oscillating signal. Typically this situation is avoided because the high pitched ringing tone hurts everyone’s ears; it can turn a crowd of nice people into an angry mob upset with the AV guy. The frequency of oscillation in this “experiment” depends on the proximity of the microphone to the speaker, the gain of the amplifier (controlled by the volume knob), and the orientation of the microphone to the chassis of the speaker.

Basic Understanding of the FFT Function

In a graphical sense, the result of performing the FFT function on a signal is a plot of power versus frequency. On the iPad screen, the green trace is voltage vs. time, measured at the output of the amplifier that feeds the speaker, and the red trace is the FFT function of the green trace: amplitude vs. frequency. The green trace shows a sinusoidal waveform with a frequency of 2.4 kHz and the red trace has a peak value at 2.4 kHz. Pictured below is a screenshot captured with iPad 2.