Phase noise greatly differs between commodity and high-performance crystal oscillators. Yet its impact can be significant, as the oscillator’s phase-noise characteristic dominates system performance. To avoid overspecifying the oscillator or over- and under-spending on this component, the engineer must understand exactly how oscillator phase noise limits system performance. An application note from Crystek Corp.’s Ramón Cerda, titled “Impact of ultralow phase noise oscillators on system performance,” offers a tutorial on phase noise and jitter.
The document starts with a basic definition of phase noise—the rapid, random fluctuations in the phase component of a signal source’s output signal—and provides the related equations. It moves on to explain the noise floor, emphasizing that the goal is to maximize the signal and minimize the noise for a high signal-to-noise ratio (SNR). Noise on a carrier is either random or deterministic. While random noise spreads the carrier, deterministic noise generates sidebands on the carrier.
When specifying spectral purity of an oscillator or signal source, one standard measurement bandwidth should be used to make any comparisons of different oscillators meaningful. After all, when the resolution bandwidth on a spectrum analyzer is changed, the noise magnitude also changes. The industry has agreed upon the “normalized frequency,” which is the correlation bandwidth for phase-noise measurements of 1 Hz.
Few spectrum analyzers have a 1-Hz resolution bandwidth—and those that do are very expensive. The tutorial explains that a spectrum analyzer will specify how close to the carrier it can measure. For measurements closer than this minimum resolution bandwidth, it is possible to normalize the reading to 1 Hz with equations and tips provided in this document.
Because a signal’s noise spectrum is symmetrical around the carrier frequency, it is only necessary to specify one side. The section closes by noting that phase noise is defined in three ways: the characteristic randomness of frequency stability; the short-term frequency instability of an oscillator in the frequency domain; and the peak carrier signal to the noise at a specific offset off the carrier.
The application note closes with a discussion of jitter, which is oscillator noise performance characterized in the time domain. It is a variation in the zero-crossing times of a signal or a variation in the signal period. As phase noise increases in the oscillator, so does jitter. It comprises a predictable component, dubbed deterministic jitter (which comes from deterministic noise), as well as random jitter (derived from random noise). The Gaussian distribution is illustrated and defined mathematically. The note closes by explaining that a true ultra-low-phase-noise oscillator (versus a commodity oscillator) uses a discrete, high-performance topology with a precision packaged crystal that has a Q greater than 70,000 for superior close-in phase noise.
Crystek Corp., 12730 Commonwealth Dr., Fort Myers, FL 33913; (239) 561-3311, www.crystek.com.