Test and Measurement, Tutorials
In Part 1 of our introduction to Spectrum Analyzers we will give an overview of what a Spectrum Analyzer is, what it does and why we use one. We will also outline some of the some of the typical measurements you may perform with a Spectrum Analyzer.
In later posts on this topic we will go into more detail on the different types of Spectrum Analyzer on the market and the key specifications and points to look for if considering a purchase. We will also be looking at the architecture of different analyzers and any limitations or problems to look out for while taking your measurements.
What does a Spectrum Analyzer do?
Normally when we consider a voltage or signal we think about how it changes or varies in time. This can be illustrated by thinking about a signal such as a square wave as in the diagram below (the red trace). As time progresses, the amplitude of the signal increases and then decreases. Normally to observe a signal’s characteristics in the time domain an Oscilloscope can be used. Oscilloscopes are designed to accurately reproduce time domain information of signals, such as glitches or rise/fall time etc. (as a basic explanation).
We know from Fourier Analysis that a signal such as a square wave is made up of a number of other signals added together. In the case of a square wave, these are multiple sine waves at harmonics of the fundamental frequency. Each of these individual signals are added together to make up the complex square wave signal that we see in the time domain. For various reasons it can be useful to observe signals, rather than in the time domain, but in the frequency domain instead.
A Spectrum Analyzer is a specialist instrument that is designed to allow us to do this, by showing us the individual frequency components of either pure sine wave signals or complex modulated signals including their harmonics, as can be seen in the diagram and screenshots below.
Typical Measurements you may make using a Spectrum Analyzer
A traditional spectrum analyzer is a scalar type instrument. By this we mean it is able to only measure the magnitude of signals and is not able to give information about the phase of a signal that it is observing.
A spectrum analyser typically has narrow IF (demodulation) bandwidth (e.g. 10MHz) and is suited to narrow band signals. Such an instrument is normally used in “frequency domain mode” and quickly sweeps over the spectrum to catch signals that occur.
Spectrum Analyzer Measurement Examples (normally considered Frequency Domain measurements)
- Search for Unknown Spectrum Components
- Evaluation on Signal Spectrum
- Channel Power
- Adjacent Channel Power / Leakage
- Noise Figure
- Intermodulation Measurements
- Scalar One Port / Two Port Measurements
- Phase Noise
More complex spectrum analyzers known as a Signal Analyzer or a Vector Signal Analyzer are able to capture information in the Time Domain and also give information about the phase of the signal being observed.
Vector Signal Analyzer’s are usually a spectrum analyser with many more features and extra hardware added. These typically have wider IF (demodulation or analysis) bandwidth (e.g. can be as little as 10MHz but normally, 28MHz, 40MHz up to even 120MHz or more) and is suited to analysis of wide band signals (communications type signals or very fast pulses). Typically these instruments are used in “time domain mode” for high rate sampling of data and performing measurements in more detail.
Vector Signal Analyzer Measurement Examples (normally considered Time Domain measurements)
- Time Domain Power
- Pulse Burst Rise / Fall Time
- Pre-Amble Measurements
- Modulation Measurement
- Modulation Accuracy (EVM / MER)
- Phase / Frequency modulation / changes over time
- Code Domain Measurements
- Signal demodulation to recover data
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