Читать книгу Handbook of Microwave Component Measurements - Joel P. Dunsmore - Страница 39

Error vector magnitude (EVM) is a figure of merit used in communications systems to describe the quality of a modulated signal compared to an idealized signal. In most cases, it is a measure in the so‐called IQ plane of the vector difference between the measured signal and the idealized signal, which is determined by recovering the modulation pattern from the measured signal and re‐creating the idealized signal. It is used when the errors are small and becomes inaccurate with large errors as the recovered signal may not be the correct signal when the EVM is quite large.

The sample point for determining the error is determined by the signaling method, and the idealized signal is time‐shifted to line up with the measured signal to find the difference at the sample point. In some signaling methods, the EVM is determined by taking the fast Fourier transform (FFT) of the modulated signal and idealized signal and measuring the vector difference in the frequency domain.

EVM is affected primarily by distortion of the channel (usually in the transmitter amplifier), nonuniform frequency response (ripples or roll‐off in the channel components), and noise in the system. For a transmitter component, which is the principal contributor to EVM, the noise contribution is generally not significant. In many modulation schemes, such as orthogonal frequency domain multiplexing (OFTM), the signal is broken into many narrow channels, such that the frequency response changes are small over each channel, and thus frequency flatness errors don't contribute to the EVM in these modulation schemes. In other cases, the measurement receiver has the ability to apply frequency response compensation, a kind‐of inverse filtering, to remove the effects of the nonideal frequency response. This is sometimes called equalization, and the EVM measurement is called equalized EVM. After equalization, the frequency response does not contribute significant errors to the EVM signal.

This leaves only distortion as the predominant contribution to EVM, and as such EVM has become a common figure of merit for distortion in these systems. EVM measurements generally require a full demodulation to evaluate the signal quality, and at this time such capabilities are not generally available in VNAs, but this is likely to change as EVM becomes a significant figure of merit in more systems.

Recently, several papers have been presented (Sombrin 2011; Freiberger et al. 2017) that demonstrate a corresponding relationship between EVM and NPR. These works are compelling and lead one to infer that with further development, the time is near when EVM can be determined without the need for full demodulation, as illustrated in Chapter 8.