Error Vector Magnitude (EVM) is a measure used to quantify the quality or performance of a modulated signal from a transmitter or receiver. In simple terms, if we consider a constellation diagram the EVM is the magnitude of the difference between the measured vector and the ideal (reference) vector. This can be visualized as below.
It can be seen from this simple diagram that EVM is influenced by a number of parameters such as below:
- Phase Error
- Frequency Error
- Magnitude Error
- Noise that contributes to all of the above
Each of these areas are contributed to not only by the signal being measured, but also by the test instrument itself which has an effect on how well it can capture the signal, but also how it is able to generate an “ideal” reference signal to use for the calculation. If we take a look at this block diagram which shows a model of transmitter EVM contributions such errors also exist during the demodulation process. Therefor the limit of EVM demodulation performance can only be as good as the error contributions added during the demodulation process in the signal analyzer.
Many of the effects we are able to correct for in DSP as part of signal synchronization etc. Although phase noise is not so easy to correct for and has a direct impact on performance.
Below are some graphics showing a visual representation of the effects of the distortions described in the model on the constellation diagram.
EVM Performance when measuring the new 802.11ac 80MHz standard 256QAM
Below are two graphics showing screenshots taken from an R&S Vector Signal Analyzer on an 802.11ac signal with a specific EVM of the transmit signal. The good signal has 33dB of EVM and the bad signal only 30dB of EVM performance. It is clear to see on the bad signal that some symbols (constellation points) are much further away from the ideal point which would result in more error and a poorer EVM result.
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Constellation with 30dB of EVM Performance
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Constellation with 33dB of EVM Performance
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Instruments with a worse EVM performance will contribute directly to this error and on signals over a wide bandwidth and such high order modulation schemes (256QAM / 1024QAM etc) EVM performance and phase noise performance of the test instrument becomes critical to measuring such parameters.
Graphics courtesy Rohde & Schwarz FSV-K70 user manual and some very helpful colleagues relating to the EVM plots above.
Posted in Test and Measurement, Tutorials, Wireless Technology
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When we talk about spectrum analysis what do we consider the most important aspects? Normally it depends on whether your background is based on time domain (scopes) or frequency domain measurements.
The scope guys will tell you bandwidth is everything so you can see the smallest glitch in a time domain signal. If you want to inspect all the detail possible in the frequency domain a spectrum analyzer is really the only tool you really should be using. The frequency domain guys will tell you its all about dynamic range (spurious free dynamic range) and phase noise performance.
Without these aspects it is not possible to get the detail required on the smallest spurs that may be created in a device under test. Or to characterise distortion performance of amplifiers with precision. Scope A/D converters have very wide bandwidth but do not have the resolution or spurious free dynamic range required to create an accurate picture of spurious and other phenomena in the frequency domain.
With this in mind, R&S has released what is seen as the highest performance spectrum analyzer ever to the market and below is an overview of what makes it so high performance, and why it is important for measurement tasks in the frequency domain.
R&S FSW Performance Overview – all the important points that make it the highest performing spectrum analyzer ever
Continue reading » » The highest performance spectrum analyzer ever – R&S FSW
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Vector Signal Analyzer
Darren Tipton of Rohde and Schwarz UK talks to Radio-Electronics.com and investigates what realtime spectrum analyzers are and how they can be used to find signals other analysers cannot see.
Spectrum analysers are devices that allow us to look at signals in the frequency domain rather than in the time domain. They are frequency selective devices that can be used to analyse characteristics of wanted signals, such as channel power, bandwidth, level and also unwanted characteristics of signals such as unwanted side band power, spurious or other interference.
In many cases a spectrum analyzer can be used to gain more information about a signal such as its phase. Such instruments are normally called vector signal analysers due to this added capability and they are able to perform for example modulation quality or frequency vs time measurements on signals by capturing the signal and post processing it.
In this short article we will discuss the differences between swept and real time spectrum analysers as well as what real time means and what advantages or disadvantages it has over traditional methods.
Continue reading » » Realtime Spectrum Analyzers – What are they?
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In our second post on spectrum analyzers we will start to talk about how a spectrum analyzer works and the principles behind the super heterodyne receiver that is used in Spectrum Analyzers to allow us to measure signals in the frequency domain.
In future posts we will look at the diagram in more detail and comment on the specification points of a Spectrum Analyzer and what sections of the system influence those specification points.
Jump to:
Part 1 » RF Spectrum Analyzer Tutorial and Basics
Part 2 » RF Spectrum Analyzer Tutorial: Super Heterodyne Swept Spectrum Analyzer and Mixer Basics
Spectrum Analyzer Architecture
In the previous post we mentioned that a spectrum analyzer is a specialist piece of equipment that allows us to view signals in the frequency domain.
Continue reading » » RF Spectrum Analyzer Tutorial and Basics – Part 2
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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.
Jump to:
Part 1 » RF Spectrum Analyzer Tutorial and Basics
Part 2 » RF Spectrum Analyzer Tutorial: Super Heterodyne Swept Spectrum Analyzer and Mixer Basics
What does a Spectrum Analyzer do?
Continue reading » » RF Spectrum Analyzer Tutorial and Basics – Part 1
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