Читать книгу Handbook of Microwave Component Measurements - Joel P. Dunsmore - Страница 83

With the advent of digitally modulated signals for RF and microwave communications, spectrum analyzers have evolved into much more complex systems that include the ability to do wideband de‐modulation of these signals. These specialized spectrum analyzers are often called vector signal analyzers (VSAs) and play an important role in component tests.

For many active components, a key figure of merit is the distortion that they apply to the vector modulated signal in the form of amplitude or phase error relative to an ideal signal. The composite of all errors over a set of digital symbols is called the error vector, and the average magnitude of this error is the error‐vector‐magnitude (EVM). While EVM is a signal figure of merit, as it is compared to an ideal waveform, the EVM from an amplifier is a combination of the EVM of the input signal and the errors added by the amplifier. From this it is clear that EVM is not a microwave‐component parameter; but a related value, residual or added EVM, is and is described as the EVM at the output signal relative to the input signal. In practice, high‐quality sources are used to produce the digitally modulated signals, so the input effect on EVM is small, but with higher data rates and wider bandwidths of modulation, these input effects are becoming more important. Thus, there is a need for a multi‐channel VSA that can compare input to output signals. A normal SA with a VSA capability does not provide such dual‐channel capability, but some manufacturers supply a specialized dual‐channel receiver for the VSA, while other implementations of a VSA use a wideband digitizer, or even a digital oscilloscope, to do direct digitization of the modulated signal. To date, up to four simultaneous channels have been reported in such a VSA.