What is phase noise and why is it important?
Phase noise is the enemy of high-quality satellite communications. It degrades signal quality, causes inter-symbol interference, and increases bit error rates. Although there is no such thing as zero phase noise, the less you have, the better. Minimizing phase noise as much as possible is critical for applications such as data communications for: military, terrestrial microwave, troposcatter, deep-space and satellite communications for ground terminals and spacecraft.
What is phase noise?
Phase noise is a term used to describe rapid, short-term fluctuations in the frequency stability of a signal. To understand it simply: Imagine you’re listening to the radio and you hear a bit of static or interference in the background. Phase noise is somewhat like that “fuzziness” that impacts the clarity and reliability of the signal.
In more technical terms, phase noise is the representation of random fluctuations in the phase (waveform) of a signal in the frequency domain. The fluctuations spread the power of a signal to adjacent frequencies, resulting in “noise sidebands” that are higher and lower than the carrier frequency – and that degrade signal quality. Phase noise is the frequency-domain representation of signal instabilities in the time domain, called jitter.
When SATCOM system engineers talk about phase noise, they are referring to the phase noise of an oscillator – “signal generators” that produce the waves used for communication. Every oscillator has some jitter in the time domain, which corresponds to some phase noise in the frequency domain. System engineers are acutely aware of the impact of any noise source and design networks to minimize phase noise as much as possible.
Time domain vs. frequency domain
The time domain shows how a signal changes over time, whereas the frequency domain shows the distribution of frequencies present in a signal. Put simply, the time domain is like watching a movie, where you see events unfold over time – while the frequency domain is like listening to music, paying attention to the different pitches and tones that make up the sound.
Phase noise vs. jitter
Phase noise and jitter both describe variations in the timing of signals – and both can affect the performance of communication systems and electronic devices – but they have slightly different meanings:
· Phase noise refers to the fluctuations in the phase of a signal over time. It is measured in decibels per Hertz (dB/Hz), representing the power of the noise at a given frequency compared to the carrier frequency. You can think of phase noise as random variations or "fuzziness" in the timing of a signal, like static on the radio.
· Jitter, on the other hand, is the deviation from the ideal timing of a signal's edges. It is measured in picoseconds (ps) or nanoseconds (ns), representing the extent of the timing variation. You can think of jitter as the "wiggling" or instability of a signal's timing, like a clock that occasionally, and unpredictably, speeds up or slows down.
Phase noise measurement
The most common way to measure phase noise is with a spectrum analyzer. The analyzer compares the signal under test to a stable reference signal. Engineers assess the power spectral density of phase fluctuations as a function of frequency offset from the carrier, typically expressed in dBc/Hz. (See FAQ at end of blog for ‘power spectral density’ definition.)
Phase noise in signal sources
Any system containing oscillators and signal sources are susceptible to phase noise. A variety of factors in a satellite link will contribute to and affect phase noise:
· Oscillator jitter
· Frequency Reference for phase locking for block upconverters (BUCs) / low noise block downconverters (LNBs)
· Phase locked loop (PLL) LNB design
· Loop bandwidth
· Power supply noise
· Grounding (electronics, cables and antenna structure)
· 20 Log(n) (where n is the frequency multiplier)
Some of these factors can be minimized through proper selection of subsystem equipment (stated/tested specs from the BUC and LNB manufacturers). Some can be mitigated through good engineering practices and careful site installation (proper grounding in particular). One of the most important contributing factors is the stability of the oscillator that is generating the timing reference for any equipment with a frequency conversion stage (BUC or LNB).
The advantage of ultra-low phase noise
Phase noise standards are implemented for good reason. By meeting or beating their specifications, you can optimize and maximize usage of the satellite resource. Well-designed Earth stations with ultra-low phase noise have the advantages of:
· Improved availability (less susceptibility to other noise sources, even atmospheric/rain)
· Increased effective coverage area
· Reduced operating costs
· Reduced antenna size
· Lower civils/capital costs
· Higher throughput (combination of data rate, modulation and coding rate)
These types of earth stations require modern LNBs with the lowest phase noise possible. If you’re looking for an LNB like this – we can help. Here’s an example of an actual Orbital Research Ka-band LNB, tested and plotted against the MIL-STD mask:
Clearly the unit passed, exceeding the standard anywhere from 5dB to 30dB. Outperforming the spec not only ensures ease of certification, it provides additional improvements to the overall satellite link. This is important to consider, whether your application requires formal certification or not.
Effects of phase noise on signal demodulation
Any noise introduced into a satellite link makes proper demodulation and decoding more difficult. The demodulated constellation of received phases gets distorted by the added noise. The pair of constellation diagrams below illustrate the case for a simple modulation scheme, QPSK:
image source: Baran, O., Kasal, M., Vagner, P., & Urbanec, T. (2012). Phase Noise Impact on BER in Space Communication. International Journal of Electrical, Electronic and Communication Sciences, 5(9).
The plot on the left is a relatively clean diagram, with the demodulated signal suffering only from random Gaussian noise (AWGN). The plot on the right shows the presence of phase noise and how it starts to spread the received constellation. With additional phase noise, the diagram will degrade into a ring of scatter points, making demodulation and decoding impossible (and producing a very high bit error rate).
As modems and satellite links push for higher modulation schemes, these constellation diagrams become ever more sensitive to the effects of phase noise.
The relevance of phase noise standards
Phase noise is typically expressed in units of dBc/Hz, and it represents the noise power relative to the carrier contained in a 1 Hz bandwidth centered at certain offsets from the carrier. For example, a certain signal may have a phase noise of -80 dBc/Hz at an offset of 10 kHz, and -95 dBc/Hz at an offset of 100 kHz.
Satellite operators, particularly those that allow access to their fleet by a wide variety of user configurations and equipment vendors, require a minimum set of standards for phase noise. A very good example of this is the U.S. DoD’s Wideband Global SATCOM (WGS) system – a nine satellite constellation operating in X-band and military Ka-band. To get authorization to operate on this fleet, earth station equipment configurations must undergo testing (ARSTRAT certification) and meet the MIL-STD-188-164C standard for phase noise. The “phase noise mask” that must be met for a satellite terminal is as follows:
Figure from MIL-STD-188-164C: Terminal Phase Noise
Testing is rigorous, time-consuming and expensive. You definitely want to pass the first time.
Lastly, a noisy reference source can undermine the performance of your LNB – no matter how good it is. You’ll get better specs by coupling your LNB with a high-stability, low phase noise oscillator. Orbital also offers a series of oscillator modules that meet this need, even on fast-moving vehicles or aircraft.
Get a free phase noise assessment
Want to know how your own system can be improved by better phase noise? Book a free 30-minute assessment with one of our experts.
FAQs:
1) If my terminal uses an external reference, how important is its quality for phase noise performance?
Very important. Even a high-performance LNB will reflect the quality of the reference driving it. A noisy or unstable reference can raise overall phase noise and compromise system performance, particularly at close-in offsets. This makes the reference oscillator a key part of the RF chain, not just an accessory.
2) Why do some systems struggle to meet phase-noise requirements even when individual components look good?
Phase noise is cumulative across the signal chain. Small issues – such as reference distribution losses, power-supply noise, grounding practices, or mismatched configurations – can add up. As a result, systems can fall short even when each component meets its standalone specification.
3) When does phase noise become a deciding factor in choosing an LNB?
Phase noise matters most when operating close to regulatory or operator limits, using higher-order modulation, or working with narrow margins. In these cases, choosing an LNB designed for low phase noise and stable LO performance can directly impact whether a system performs reliably in real operating conditions.
4) What is power spectral density?
Power spectral density (PSD) describes how the power of a signal or noise is distributed across frequency. In the context of phase noise, it shows how much phase-noise power exists at different frequency offsets from the carrier, normalized to a 1 Hz bandwidth. This is why phase noise is expressed in units of dBc/Hz: it allows engineers to compare noise performance at specific offsets and understand how phase noise will affect demodulation, adjacent channels, and overall system performance.
5) Is phase noise mainly a concern for advanced or specialized satellite applications?
While phase noise is always present, its impact grows as systems become more demanding. Applications involving higher data rates, tighter masks, or constrained link budgets are more sensitive, making phase noise an important consideration in professional and mission-critical SATCOM environments.
6) How should phase-noise requirements influence early system design decisions?
Phase noise is easiest to manage when it’s considered early in system design. Choices around modulation schemes, reference architecture, frequency planning, and margin targets can all affect how sensitive a system will be later on. Addressing phase noise upfront helps avoid costly redesigns or last-minute performance compromises during integration or testing.