A Beginning Tutorial on Spectrum Analysis (Part-1)

This blog is intended to be a beginning tutorial on spectrum analysis. It has written for those who are unfamiliar with spectrum analyzers and would like a basic understanding of how they work, what you need to know to use them to their fullest potential, and how to make them more effective for particular applications.

What is Spectrum Analyzer?

If you are designing, manufacturing, or doing field service/repair of electrical devices or systems, you need a tool that will help you analyze the electrical signals that are passing through or being transmitted by your system or device. By analyzing the characteristics of the signal once it’s gone through the device/system, you can determine the performance, find problems, troubleshoot, etc.

How do we measure these electrical signals to see what happens to them as they pass through our device/system and verify the performance? We need a passive receiver, meaning it doesn’t do anything to the signal — it just displays it in a way that makes it easy to analyze the signal. This is called a spectrum analyzer. Spectrum analyzers usually display raw, unprocessed signal information such as voltage, power, period, wave shape, side-bands, and frequency. They can provide you with a clear and precise window into the frequency spectrum.

Frequency vs Time Domain

Traditionally, when you want to look at an electrical signal, you use an oscilloscope to see how the signal varies with time. This is very important information; however, it doesn’t give you the full picture. To fully understand the performance of your device/system, you will also want to analyze the signal(s) in the frequency domain. This is a graphical representation of the signal’s amplitude as a function of frequency The spectrum analyzer is to the frequency domain as the oscilloscope is to the time domain. (It is important to note that spectrum analyzers can also be used in the fixed-tune mode (zero spans) to provide time-domain measurement capability much like that of an oscilloscope.)

Frequency vs Time Domain

The figure shows a signal in both the time and the frequency domains. In the time domain, all frequency components of the signal are summed together and displayed. In the frequency domain, complex signals (that is, signals composed of more than one frequency) are separated into their frequency components, and the level at each frequency is displayed. Frequency domain measurements have several distinct advantages. For example, let’s say you’re looking at a signal on an oscilloscope that appears to be a pure sine wave. A pure sine wave has no harmonic distortion. If you look at the signal on a spectrum analyzer, you may find that your signal is actually made up of several frequencies. What was not discernible on the oscilloscope becomes very apparent on the spectrum analyzer.

Swept Analyzer

Swept Analyser

The most common type of spectrum analyzer is the swept-tuned receiver. It is the most widely accepted, general-purpose tool for frequency-domain measurements. The technique most widely used is superheterodyne. Heterodyne means to mix — that is, to translate frequency — and super refers to super-audio frequencies or frequencies above the audio range. Very basically, these analyzers “sweep” across the frequency range of interest, displaying all the frequency components present. We shall see how this is actually accomplished in the next section. The swept-tuned analyzer works just like the AM radio in your home except that the dial controls the tuning on your radio and instead of a display, your radio has a speaker.

FFT Analyzer Block Diagram

FFT Analyser Block Diagram

The Fourier analyzer basically takes a time-domain signal, digitizes it using digital sampling, and then performs the mathematics required to convert it to the frequency domain*, and display the resulting spectrum. It is as if the analyzer is looking at the entire frequency range at the same time using parallel filters measuring simultaneously.

It is actually capturing the time domain information which contains all the frequency information in it. With its real-time signal analysis capability, the Fourier analyzer can capture periodic and random, and transient events. It also can provide significant speed improvement over the more traditional swept analyzer and can measure phase as well as magnitude. However, it does have its limitations, particularly in the areas of the frequency range, sensitivity, and dynamic range. We shall discuss what these terms are and why they are important in a later section.

Fourier analyzers are becoming more prevalent, as analog-to-digital converters (ADC) and digital signal processing (DSP) technologies advance. Operations that once required a lot of custom, power-hungry discrete hardware can now be performed with commercial off-the-shelf DSP chips, which get smaller and faster every year. These analyzers can offer significant performance improvements over conventional spectrum analyzers, but often with a price premium.

Read the next part here.