# EVM Measurements-EVM vs subcarrier measurement,EVM vs symbol measurement

This page compares EVM vs subcarrier and EVM vs symbol and mentions **difference between**
EVM vs subcarrier measurement and EVM vs symbol measurement with respect to WLAN/WiMAX system
as per OFDM modulation.

As we know EVM is the short form of Error Vector Magnitude. It is also referred as RCE (Relative Constellation Error). It is obtained by computing vector from mapping position obtained for received symbols to ideal mapping position on the constellation diagram (IQ diagram). It displays rms value of EVM. This is shown in the figure-1.

This **EVM measurement** is useful for performance analysis of modem under
various impairments and imperfections e.g. Local Oscillator Stability,
filter compression, DAC/ADC, symbol rates and interfering signals etc.

Moreover high PAPR requires highly linear PA (Power Amplifier). Due to this facts, it is advisable to have EVM measurement so that linearity and efficiency can be verified.

The figure-2 mentions EVM formula. In WLAN system, one OFDM symbol consists of total 64 subcarriers. This 64 subcarriers made of 48 data subcarriers, 4 pilot subcarriers, 1 DC, 6 left guard and 5 right guard subcarriers. Suppose there are 20 such symbols in a frame of duration 10ms with bandwidth of 20MHz.

## Error Vector Spectrum-EVM vs subcarrier measurement

The figure-3 depicts **EVM vs subcarrier measurement**. As shown X-axis is subcarriers and Y-axis is EVM.
Here 52 total used subcarriers are plotted on X -axis and EVM for these respective 52 points are plotted
on Y-axis. This type of plot is also known as Error Vector Spectrum.

## Error Vector Time-EVM vs symbol measurement

The figure-4 depicts **EVM vs symbol measurement**. As shown X-axis is symbols and Y-axis is EVM.
Here 20 symbols are shown on X-axis and EVM for all the symbols i.e. 52 points per symbol which gives
total 1040 points on the plot. Figure shows EVM measurement for all the subcarriers of symbol number-5.
Above mentioned EVM formula will be used after individually determining EVM values of all 52 subcarriers
in order to calculate aggregate EVM (rms) value for one OFDM symbol.
This type of plot is also known as Error Vector Time.

## Links to useful Measurements

Following are the links to other useful measurements available on Test and Measurement World website.

Difference between EVM vs subcarriers and EVM vs symbols

Current measurement vs Voltage measurement

Inductance measurement vs Capacitance measurement

Difference between Power and Energy measurements

EMC measurement vs EMI measurement

Electric and Magnetic Field Measurements

## What is difference between or comparison between

Following links mention difference or comparison between various equipments:

comparison between VSG and VSA

DIfference between SNA ans VNA

Oscilloscope Bandwidth versus Rise time

Oscilloscope vs Logic Analyzer

Difference between Oscillscope types

Comparison between spectrum and network analyzer