What Is the Error Vector Magnitude?

Radio signals are often analyzed by tracking something called an error vector. This is typically the difference between the strength of the signal measured, and one of ideal intensity, called the reference. Generally used to plot both signals, a chart called an I-Q plane comprises a vertical axis labeled Q and a horizontal axis labeled I; each letter corresponds to a component of the signal. An error vector, the numerical difference between both signals, can feature an average power called the Error Vector Magnitude (EVM).

Often used to measure the performance of radio electronics, EVM is typically expressed as a ratio. The power of the error vector, along with a mathematical average of the reference power, is generally used to calculate the Error Vector Magnitude. This is often called Receive Constellation Error (RCE). Signal quality is often represented by the graphical placement of constellation points, but imperfections can occur for various reasons. The EVM generally measures how far these points are from their intended places.

Error Vector Magnitude is typically measured for digital radio receivers and transmitters. The equipment used to measure it can process a signal similar to how a radio component called a demodulator does. Calculations are then performed by the measurement system. These measurements are often used to identify what kind of signal degradations are happening; sometimes the source of the signal problem can be identified as well.

A ratio of average power to peak power in a single carrier system, Error Vector Magnitude can also include a ratio of two average powers in multi-carrier radios, in which the signal interaction can be a little more complex. Modulation error ratio in multi-carrier systems generally represents the ratio of average signal power to average significance of an error. Under certain circumstances, it is related to EVM.

Specialized software can be used to measure Error Vector Magnitude. A number of software programs are able to determine signal integrity, and can include pass/fail indicators to show if measurements meet particular criteria. Various other mathematical calculations related to signal characteristics are often performed as well. On a computerized graph, symbols and letters can be generated to denote things like the location of the measured signal and the point where the reference signal should be. Physical angles can be used to calculate the magnitude of the error, but this doesn’t always find the nature of the problem, even with the help of a computer.