Fourier Transform—A Fourier transform is a graphical representation of all frequency components that appear in a signal over a given period of time. The x-axis of a Fourier transform is in units of frequency. The y-axis of a Fourier transform shows the amplitude of the signal at a given frequency.
Figure 1. Many of the important terms to understand with frequency performance can be visualized together on a Fourier transform.
Amplitude—Amplitude refers to the voltage or current level of signals at a given frequency. In a Fourier transform, the amplitude refers to the height of the histogram in that particular frequency bin. Power can also be used to describe the level of a given frequency or a integration of amplitudes over a given frequency range.
Fundamental Frequency—Also known as the carrier frequency, the fundamental frequency is the lowest frequency component of a periodic signal. In an ideal Fourier transform of a sine wave, only the fundamental frequency would appear. Most of the specifications below are generated using an almost ideal sine wave.
Harmonics – Harmonics, or harmonic frequencies, are frequencies that are integer multiples of the fundamental frequency. Harmonics are often not part of the actual signal, but appear due to Nyquist sampling theory and transmission line reflections. An ideal square wave will appear as a set of all harmonics of the fundamental frequency on a Fourier transform.
Spurs—Spurs, also known as device spurs, are frequency components that appear in signals because of the electrical components of the instrument. Some examples of spurs are interleaving anomalies in analog-to-digital converters (ADCs), leakage of oscillator clock signals, and power inserted by amplifiers.
Noise—All voltage and frequency components that are not present in the actual or ideal signal, spurs, or harmonics, but are present in the measurement or generation of test signals, are noise. Some common causes of noise are environmental magnetic fields, temperature, and ground loops. The noise floor is the maximum Fourier transform amplitude of any noise in the device’s frequency range.
Front End—The front end of an instrument is all components external to the converter including analog amplifiers and filters.