The Ultimate Guide to Understanding Phase Noise

Robert Rutkowski - Bliley Technologies, Inc.

Feb 8, 2017

What is Phase Noise?


To begin understanding phase noise, here are some basic definitions of Phase Noise and what is known as Jitter. 
 
Phase Noise- The frequency domain representation of rapid, short-term, random fluctuations in the phase 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 Path to Understanding Phase Noise
 
To build an in-depth understanding of phase noise, we'll use a simple 5 step process. Once you understand the 5 steps, you'll understand what makes phase noise...well... phase noise! 

 

I bet you're wondering how Spectral Density is connected to Phase Noise in the 5 step process. Here's some more details on each step:
 
Step 1: Spectral Density- 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:

You are now looking at what is known as the Signal Power Density (Step 2) of the noise. 

Now focus only on the upper sideband of the graph from fstart to fstop called the "Single Side Band". Check out the illustration below:

Step3: 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: 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 4). 
 
Step 5: 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 smaller 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 (Step 5).
 
So...
SSB Noise Density = Phase Noise
 
...and that's where phase noise comes from! Easy, right?
         
What are Some Causes of Phase Noise?
Here are some common sources of phase noise in crystal oscillators caused by vibrations (jitter). 
 
Random Noise Sources:
What's Next?
Phase noise is considered one of the most important aspects of an oscillator's performance. Therefore, it is important to understand an in-depth look at what phase noise is and some of its common causes.
 
Now that you know all about phase noise and jitter, you'll have an easier time selecting the best output signal type in your oscillators.  After all, the consideration of phase noise differs from application to application.

 

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