How Signal Generator Spectral Purity Affects Measurements

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What is spectral purity?

Spectral purity is the inherent stability of a signal. Does it change in frequency? Stabilities can either be short or long term. Drift or long-term stability is usually defined over a period of time greater than a second. Does the signal drift off frequency in minutes, hours, days, or months? Current signal generator technology generally offers good long-term stability.

The greater concern is for short-term stability or changes in frequency in less than one second. These fluctuations come from non-deterministic signals like noise, shot noise and 1/f flicker noise that modulates the carrier. These affect both phase and amplitude.