Maintaining strong frequency stability in electronic RF circuits by eliminating phase noise is important in many high-end communication applications. This is especially true for precise targeting in radar systems and spectral purity in other communication systems.
Let's take a deep dive into exactly what phase noise and jitter mean. This will help give you a better idea as to why reducing a system's phase noise is significant.
What Is Phase Noise?
is represented in the frequency domain
of a waveform and consists of rapid, short-term, random fluctuations in the phase (frequency). This is caused by time domain instabilities (jitter).
Be sure not to confuse phase noise with 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!
"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:
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".
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 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
SSB Noise Density = Phase Noise
...and that's where phase noise comes from! Easy, right?
What Causes Phase Noise?
Phase noise in higher-end applications (such as radar communications, military communications, and space & satcom communications) is typically caused by
- High vibrations
- Micro vibrations
- g forces and acceleration sensitivity
anti-vibration, g-sensitivity crystal oscillator
is the best way to eliminate phase noise from all of these potential sources.