SiTime Corporation
03/27/2024 | News release | Distributed by Public on 03/28/2024 01:17
A Closer Look at Phase Noise Plots to Identify the Source of Phase Jitter
Phase jitter is a crucial factor to consider when assessing the performance of electronic devices. It refers to the variation in the phase of a signal, which can lead to timing errors and affect overall system integrity. To better understand and minimize phase jitter, engineers often rely on phase noise plots. These plots provide a visual representation of the different noise sources influencing phase modulation.
In this post and in the video below, we present a short "4-minute clinic" on how to determine the dominant source of jitter by inspecting phase noise plots. Here we discuss a technique to analyze phase noise plots and identify the main source of jitter. This technique allows for targeted improvements in design by identifying where to focus efforts to minimize phase jitter. The goal of this 4-minute clinic is to be able to prioritize design optimization that have the highest impact on reducing phase jitter. You'll also find a list of additional resources below to gain further insights into this important topic.
But first, what is phase jitter and why is it important? Phase jitter is a crucial factor to consider when assessing the performance of electronic devices. It refers to the variation in the phase of a signal, which can lead to timing errors and affect overall system integrity. To better understand and minimize phase jitter, engineers often rely on phase noise plots. These plots provide a visual representation of the different noise sources influencing phase modulation.
Learn which offset frequency region dominates the integral of phase noise, by inspection.
The Anatomy of a Phase Noise Plot
Phase noise plots represent the variation in phase modulation in the frequency spectrum, with different regions indicating different sources of noise. An ideal sine wave has no noise and appears in the frequency spectrum as an impulse function. However, when a phase error term is introduced, phase modulation occurs, resulting in a spectral broadening around the impulse function. By zooming in on a single sideband and normalizing the Y-axis to dBc/Hz (decibels relative to the carrier per Hertz), we obtain a phase noise plot. This plot highlights three distinct regions where noise sources can be grouped.
The sources of phase noise can be grouped into three key regions.
Analyzing the Dominant Noise Sources
To identify the dominant source of phase jitter, we need to inspect the phase noise plot in more detail. One useful technique involves drawing a -10 dB per decade line above the plot. By gradually lowering this line until it intersects the phase noise curve, we can pinpoint the region that dominates the integral. This region represents the noise source contributing the most to phase jitter. The key lies in observing which part of the phase-noise curve appears above the -10 dB/dec reference line first.
Easily identify which region dominate the integral by finding the area that intersects and passes through the phase noise curve.
Applying the Technique
Let's consider a few application examples to illustrate how this technique can help us identify the dominant noise sources in different systems. In the first scenario shown in the plot below, where a phase lock loop is employed to synthesize a new output frequency, we find that the region dominated by the loop is the most significant source of phase jitter. Therefore, improving the other areas, such as oscillation circuitry and the noise floor, would yield minimal impact.
Phase noise plot with the dominate source of phase noise in the PLL region.
In another application, as shown below, an oscillator with an offset frequency range of 100 Hz to 3 MHz reveals that the far-out offset frequency region is the primary source of phase jitter. In this case, focusing on reducing noise in the output buffer can significantly enhance the design.
Phase noise plot with the dominate source of phase noise in the Broadband noise region.
Similarly, for a system concerned with a frequency range between 1 Hz and 10 kHz, the close-in phase noise dominates, indicating that minimizing noise in the resonator oscillator circuit combination and the environment is crucial.
Phase noise plot with the dominate source of phase noise in the timing reference region.
Through a simple yet insightful inspection of phase noise plots, engineers can effectively determine the dominant sources of phase jitter. By isolating the region of interest in the plot, drawing a -10 dB projected line above it, and identifying the point of intersection, we gain valuable insights into the primary contributors to phase jitter. Armed with this knowledge, engineers can direct their efforts and resources towards optimizing the specific noise sources that have the most significant impact on system performance.
4-Minute Clinic: Determine the Dominant Source of Jitter by Inspection of Phase Noise Plot
Want to know more about where best to spend your time and resources to reduce jitter and improve your design? Watch this short video that covers this topic in more depth.
Go beyond the 4-minute clinic in this application note that discusses how to apply the three-step procedure, how to account for the bandwidth that "views" only a portion of the phase noise plot, how to factor for spurious noise, and how to better understand sources of noise. You will find a mathematical basis for the procedure, which extends its application beyond phase noise to include logarithmic plots of other quantities for power and fields.
Additional Resources
For more information and in-depth learning opportunities, visit learning.sitime.com and check out the resources below.
Need practical knowledge of phase noise? This course builds insight for understanding and working with phase noise in any system. It defines phase noise and where it comes from, explains how to interpret plots by visual inspection and convert phase noise into jitter, plus discusses equipment and best practices for measuring phase noise.
Learn how to best use a phase noise analyzer for the most accurate phase noise measurements. This step-by-step video tutorial provides tips on how to set up an analyzer, use the features and settings, and understand results.
This application note provides a theoretical overview of phase noise and then focuses on practical phase noise measurement recommendations such as properly connecting a signal under test to the instrument, setting up the phase noise analyzer, and choosing appropriate settings.
This application note describes test methodologies to measure the true performance of ultra-low jitter oscillators using differential measurements.
Find a hosts of online and downloadable jitter and phase noise calculators to speed analysis and assist in your design estimation needs. Use these calculators to compute, convert or evaluate jitter and phase noise.
Each new generation of technology must conquer the effects of jitter. To do so, jitter and its causes must be understood. Learn the three most common types of jitter, how to measure it, and get valuable resources for a deeper understanding.
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