Spectral purity is an important characteristic, as it represents a signal’s inherent stability. Furthermore, stability can be defined as either long-term or short-term. Signal generators must be capable of generating spectrally pure signals to allow for effective measurements. This topic is the subject of a new application note from Keysight Technologies titled “Signal Generator Spectral Purity.”
The application note first defines both long-term and short-term stability. Long-term stability, or drift, is usually defined over a period of time greater than one second. Current signal generator technology generally allows for good long-term stability, according to the document. Short-term stability is of greater concern. It is a result of fluctuations from non-deterministic signals, such as shot noise and flicker noise. This noise effectively modulates a carrier, affecting both amplitude and phase.
Short-term stability is commonly depicted as a plot of the single-sideband (SSB) phase noise in a 1-Hz bandwidth versus frequency offsets from the carrier. SSB phase noise is expressed as decibels-relative-to-the-carrier (dBc). In addition to SSB phase noise, spurious signals and residual FM are both described.
The importance of spectral purity for mobile radio is discussed. Essentially, the spacing between radio channels is decreasing due to the scarcity of available spectrum. Thus, designers must ensure that receivers attain greater selectivity. Measuring the selectivity of a receiver requires a signal generator that can generate spectrally pure signals. Furthermore, the application note illustrates an adjacent channel selectivity measurement, which is often performed to determine how well a receiver can reject unwanted signals.
An illustration shows how a signal generator with inadequate phase noise performance can effectively nullify measurements. After mentioning the hum and noise measurement, the application note also explains why it is important for a signal generator to generate spectrally pure signals when it is used it as a local-oscillator (LO) substitution in a test setup. Another point that is mentioned is how current radar systems demand better target resolution. Thus, spectrally pure LOs/signal generators are required.
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