Summary of FFTs and oscilloscopes: A practical guide
This article explains the practical application of Fast Fourier Transform (FFT) in digital oscilloscopes, transforming time-domain data into frequency-domain analysis. It highlights how FFT reveals signal bandwidth and spectrum details that are difficult to discern visually in the time domain, using an amplitude-modulated carrier example to demonstrate its effectiveness in measuring bandwidth accurately.
Parts used in the FFT Oscilloscope Analysis:
- Digital oscilloscope
- Fast Fourier Transform (FFT) algorithm
- Discrete Fourier Transform (DFT)
- Amplitude-modulated carrier signal
- Trapezoidal pulse modulation function
- Cursors for measurement
The FFT (Fast Fourier Transform) first appeared when microprocessors entered commercial design in the 1970s. Today almost every oscilloscope from high-priced laboratory models to the lowest-priced hobby models offer FFT analysis. The FFT is a powerful tool, but using it effectively requires some study. I’ll show you how to set up and use the FFT effectively. We’ll skip the technical description of the FFT, because its already implemented in the instruments. Instead I’ll focus on the practical aspect of using this great tool.
The FFT is an algorithm that reduces the calculation time of the DFT (Discrete Fourier Transform), an analysis tool that lets you view acquired time domain (amplitude vs. time) data in the frequency domain (amplitude and phase vs. frequency). In essence, the FFT adds spectrum analysis to a digital oscilloscope.
If you look at upper trace in Figure 1, you’ll see an amplitude-modulated carrier that uses a trapezoidal pulse as the modulation function. If you look at the time-domain view in Fig. 1 and I ask you to tell me the bandwidth of the signal, you’d have a hard time. But take the FFT of this signal and you get another point of view. The signal has a linearly swept frequency and the bandwidth, marked by the cursors, is 4.7 MHz. That’s how the FFT adds to the capability of the oscilloscope, it provides another point of view for the same data.
Figure 1. The time domain view in the top grid shows a pulse modulated RF carrier while the frequency domain view in the lower grid shows a uniform distribution of the carrier frequency between 997 MHz and 1002 MHz.
Read more: FFTs and oscilloscopes: A practical guide
- When did the FFT first appear in commercial design?
The FFT first appeared when microprocessors entered commercial design in the 1970s. - Can almost every oscilloscope offer FFT analysis today?
Yes, almost every oscilloscope from high-priced laboratory models to the lowest-priced hobby models offers FFT analysis. - What does the FFT algorithm reduce?
The FFT is an algorithm that reduces the calculation time of the Discrete Fourier Transform (DFT). - How does the FFT change the view of acquired data?
The FFT lets you view acquired time domain data in the frequency domain, adding spectrum analysis to a digital oscilloscope. - Why is it hard to tell the bandwidth from the time-domain view?
In the time-domain view of an amplitude-modulated carrier with a trapezoidal pulse, telling the bandwidth is very difficult without the FFT. - Does the FFT provide a different perspective on the same data?
Yes, the FFT provides another point of view for the same data by showing the frequency distribution. - What specific bandwidth value was measured in the example?
The bandwidth marked by the cursors in the example was 4.7 MHz. - What frequency range does the lower grid show in Figure 1?
The frequency domain view shows a uniform distribution of the carrier frequency between 997 MHz and 1002 MHz.
