Inside Frequency Control

Ultimate Guide to Understanding Phase Noise

Posted by Bliley Technologies on Jun 14, 2016 9:00:00 AM

Ultimate Guide to Phase Noise

What Is Phase Noise?

To begin understanding phase noise, it's important to understand the difference between phase noise and jitter.
Phase Noise is represented in the frequency domain and consists of rapid, short-term, random fluctuations in the phase (frequency) of a waveform, caused by time domain instabilities (jitter).
Jitter is a method of describing the stability of an oscillator in the Time Domain. It combines all the noise sources together and shows their effect with respect to time.

In the simplest terms, phase noise describes the stability of an oscillator in the Frequency Domain while jitter describes stability in the Time Domain. 

A Simple 5-Step Path to Understanding Phase Noise

To build an in-depth understanding of phase noise, try using this simple 5 step process. Once you understand the 5 steps, you'll understand what makes phase noise...well... phase noise! 
 
Understanding Phase Noise
 
"How does Spectral Density connected to Phase Noise in the 5 step process?" you ask? Here's some more details on each step.

Step 1: Spectral Density

Spectral Density is a measure of a signal's power intensity in the frequency domain. The spectral density provides a useful way to characterize the amplitude versus frequency content of a random signal.

Step 2: When plotting each spectral density point at varied frequency intervals of your choosing (In this case every 1Hz), you're left with a graph that looks like this:Signal Power Spectral Density

Step 2: Signal Power Density

You are now looking at what is known as the Signal Power Density of the noise. 

Now focus only on the upper sideband of the graph from fstart to fstop, this is called the "Single Side Band".

Single Side Band

Step 3: Noise Power Density

We can now refer to the plotted part of the single side band as noise (Anything above the nominal oscillator frequency (Fosc) and not harmonically related can be considered phase noise). The technical term for this part of our graph is Noise Power Density (Step 3). We measure noise power density in dBW (LOG(Watts)) at this point because of the large range which we are looking. 

Step 4: SSB Noise Density

When we combine the single side band and Noise Power Density, we are actually measuring what's called SSB (Single Side Band) Noise Density

Step 5: Phase Noise

Finally, we can look at this in the time domain and we see a "jittery" waveform (see graph), we are looking at "jitter". Because the jitter is much Jittersmaller than one complete period (see graph), we can say it is caused by "Phase Fluctuations" (instead of frequency fluctuations). Since these fluctuations are noise, it's actually phase noise.
 
So...
SSB Noise Density = Phase Noise
...and that's where phase noise comes from! Easy, right?

What Causes Phase Noise?

Here are some common sources of phase noise in crystal oscillators caused by vibrations (jitter). 
Random Noise Sources:
Related: Causes and solutions for phase noise in high-end radar and communication systems.

What's Next?

Achieving low phase noise in a crystal oscillator is critical to achieving high-performance. Therefore, it's important to understand an in-depth look at phase noise. Instantly download our free visual guide to the impact of phase noise on various applications.
 
Now that you know all about phase noise and jitter, you might find it useful to learn about the best output signal type in your oscillators.
Download our visual guide to the effects of phase noise

Topics: phase noise, OCXO, engineering