# SIGNAL CHAIN BASICS #56: Clock Jitter Demystified—Random Jitter and Phase Noise

John Johnson, Manager, Market Development and Systems Engineering, Texas Instruments discusses the relationship between jitter and phase noise and how they impact the performance of high-speed links.

(Editor's note : Signal Chain Basics is an ongoing (and popular) series; you can click here for a complete, linked list of installments #1 through #55 of the series, and here for all installments.)

In previous articles we discussed clock jitter basics, including the components comprising total jitter and how jitter impacts the performance of high-speed links. In this installment we discuss the relationship between jitter and phase noise. This lays the foundation for future discussions on clocking data converters.

Time domain and frequency domain

Figure 1 depicts the nature of the information provided by measuring a signal in the time domain vs. the frequency domain. Both provide insight into the content of the signal and possible approaches to optimize the signal-to-noise ratio (SNR). It is important to understand where in the frequency spectrum the noise content of a signal resides because a system is susceptible to performance degradation due to noise within a specific bandwidth. Frequency domain measurements provide this insight.

Figure 1 : Time domain vs. frequency domain measurements.

What is phase noise?

Phase noise is a frequency domain measurement that is the power spectral density of a signal’s phase. To better understand the definition of phase noise, let’s consider how it is measured. Figure 2 shows a typical phase noise measurement setup in which a clock oscillator is connected to a spectrum analyzer.

Figure 2: Phase noise measurement.

A phase noise measurement has the following characteristics:

1. The spectrum is considered symmetrical about the frequency (f C), therefore, only half (one side) of the spectrum is evaluated. This is called ‘single-sided’ phase noise.
2. It is measured within a 1 Hz bandwidth. It is assumed that the power level is constant within this bandwidth. Therefore, phase noise is a power spectral density.
3. It is measured relative to the signal’s power at frequency f C and is expressed in dBc/Hz.
4. It is measured at various frequency offsets
Design category:

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