Читать книгу Handbook of Microwave Component Measurements - Joel P. Dunsmore - Страница 81

Another class of signal sources are vector signal generators, which have an internal I/Q modulator that allows an almost infinite variety of signals to be created. Some of these vector sources (also called digital sources because they can create signals using digital modulation techniques) have built‐in arbitrary‐waveform generators (AWGs, or arbs in the vernacular), while others have broadband I/Q inputs to allow external AWGs to drive their vector modulators directly.

With vector sources, the AWGs can be used to create a wide variety of signals including extremely fast switching CW sources (within the bandwidth of the I/Q modulator), two‐tone or multitone signals, pseudo‐random noise waveforms, and complex modulated signals following the formats used in digital communications and cellular phones.

Some key attributes of vector sources are the modulation bandwidth of the I/Q inputs, the modulation bandwidth or speed of the arbitrary waveform generator (if it is built‐in), the memory of the AWG (which affects the length of signals that can be created), and the I/Q fidelity or linearity of the modulator. This linearity limits the ability of the vector source to produce clean signals. For example, a two‐tone signal can be created by doing a double sideband suppressed carrier modulation, but if there is imbalance or non‐linearity in the modulator, there will be carrier leakage between the two tones.

The output power amplifiers of vector sources are important as their distortion will directly affect the modulated signal, causing TOI, and the spectral spreading of modulated signals.