Time Domain and Frequency Domain Measurements

Measurements made in the time domain are the basis of all measurements in the 89600 VSA software. The time domain display shows a parameter (usually amplitude) versus time. You are probably familiar with time domain measurements as they appear in an oscilloscope. Similar measurements may be viewed with the time measurement data capability.

Frequency-domain displays show a parameter (again, usually amplitude) versus frequency. A spectrum analyzer takes an analog input signal—a time-domain signal—and uses the Fast Fourier Transform (FFT) to convert it to the frequency domain. The resulting spectrum measurement shows the energy of each frequency component at each point along the frequency spectrum.

Many signals not visible in the time domain (such as noise and distortion products) are clearly visible in the frequency domain. Because spectrum displays show frequency components distributed along the frequency axis, it's possible to view many different signals at the same time. This is why the spectrum analyzer is such a useful tool for looking at complex signals—it lets you easily measure (and compare) the frequency and amplitude of individual components.

The Y axis (amplitude)

Time-domain measurements are usually viewed with a linear X axis and a linear Y axis (think of an oscilloscope). Frequency-domain measurements are sometimes viewed with a linear Y axis and a linear X axis, but usually must be viewed with a logarithmic Y axis, because this is the only way to view very small signals and much larger signals simultaneously.

Let's look at the spectrum of a sine wave. Because the amplitude of any harmonic is small relative to the fundamental frequency, it's nearly impossible to view a harmonic on the same display as the fundamental unless the Y-axis scale is logarithmic. That's why most measurements made with spectrum analyzers use a logarithmic amplitude scale—a scale based on decibels. And because the dB scale is by definition logarithmic, there's no need to use logarithmically-spaced graticule lines.

The X axis (frequency)

Sometimes it's convenient to use a logarithmic X axis. Perhaps most familiar to you is the frequency response measurement. This is traditionally displayed with a log X axis (frequency) versus a log Y axis (relative magnitude).

But most measurements do not require a logarithmic frequency scale. In fact, when making spectrum measurements it's easier to characterize harmonics with a linear X-axis scale because harmonics that are multiples of the same fundamental will appear at evenly spaced intervals.