Introduction
Phase noise measurement is a critical aspect of evaluating the performance of oscillators, signal generators, and other RF systems. Phase noise refers to the frequency stability of a signal, often quantified as random deviations or “jitter” in the phase of the signal.
In this guide, we explore the concept of phase noise, its importance in modern electronics, how to measure it effectively, and the tools available. Understanding phase noise measurement ensures that RF and microwave systems meet stringent requirements for communication, radar, and timing applications.
Key Takeaways
- Phase noise measurement calculates the short-term frequency stability of a signal.
- Accurate measurement ensures the reliable performance of critical RF systems.
- Multiple techniques, such as direct spectrum analysis, time domain analysis and cross-correlation methods, are used for phase noise evaluation.
- Tools like spectrum analyzers and phase noise testers play a pivotal role in phase noise measurement.
- Addressing excessive phase noise improves system performance in high-precision communication and radar systems.
What is Phase Noise?
Phase noise refers to unwanted, short-term fluctuations in the phase of a waveform. It is an inherent characteristic of any fixed or tunable frequency source, affecting signal clarity and stability. It is particularly important in systems requiring high precision, such as satellite communications and radar systems.
Phase noise is typically measured as the noise spectral density in a 1 Hz bandwidth at an offset frequency of interest from the carrier frequency. A source’s phase noise characteristics are symmetrically below and above the carrier frequency or lower and upper sidebands, respectively, although phase noise is usually measured within the upper single sideband (SSB).
Phase noise can be measured using specialized equipment such as phase noise analyzers and spectrum analyzers.
History and Evolution of Phase Noise Measurement
Phase noise measurement has evolved significantly:
- Early Techniques – Initial methods in the 1950s and 1960s involved manually tuning filters and recording deviations with rudimentary instruments. For example, early radar systems used analog techniques to measure deviations, and oscillators were tested using hand-calibrated setups. The 1970s saw improvements with the advent of basic digital tools for more consistent results.
- Advancements in Spectrum Analysis – The introduction of spectrum analyzers in the 1970s revolutionized measurements. For instance, Hewlett-Packard introduced the HP 8566A spectrum analyzer in 1978, which became a milestone for RF and microwave signal analysis, offering high dynamic range and precision.
- Modern Tools – Today, a variety of methods are available for phase noise measurement. The optimal use cases for these methods vary greatly and may require specialized equipment such as high-end oscillators or phase detectors. A new solution from Signal Hound employs the cross-correlation method while utilizing two spectrum analyzers and a dedicated phase noise tester to match the high-sensitivity of dedicated equipment across a wide frequency range, while remaining cost-effective.
Why Phase Noise Measurement Matters
Phase noise affects system performance in several ways:
- Communication Systems – Degraded phase noise leads to higher bit error rates, as seen in modern wireless communication systems like 4G and 5G networks. For example, the high precision required in LTE and 5G transmissions makes phase noise a critical factor in determining data integrity and reducing transmission errors.
- Radar Systems – Poor phase noise performance reduces resolution and accuracy, as demonstrated in advanced radar systems like the AN/APG-77 used in the F-22 Raptor, which demands extremely low phase noise to achieve high target resolution and precision. Studies from the early 2000s show that reducing phase noise significantly improved detection ranges in military applications.
- Timing Applications – Phase noise directly impacts synchronization and timing precision, such as in GPS systems, where low phase noise is essential for maintaining nanosecond-level timing accuracy, enabling precise location tracking. For instance, modern atomic clocks used in GPS satellites, first introduced in the 1990s, depend on ultra-low phase noise oscillators to ensure reliability.
The level of phase noise impacts the overall dynamic range and measurement sensitivity of RF systems. High phase noise can reduce the performance of devices such as oscillators and frequency synthesizers, crucial in RF and microwave applications.
Techniques for Measuring Phase Noise
- Direct Spectrum method uses a spectrum analyzer to directly analyze the frequency spectrum of the signal. This is a quick method of testing and quite simple in comparison with other methods. However, the sensitivity and measurement accuracy can be limited by the spectrum analyzer’s own noise floor.
- Time Domain analysis generally requires a high-end oscilloscope or time interval analyzer. This method is well-suited for broadband noise and analyzing wideband signals and applications where time stability is important. Time domain analysis can lack sensitivity for low phase level measurements and has limited offset resolution which is often required for RF and microwave applications. Additionally, high-end oscilloscopes are usually quite costly.
- The cross-correlation method employs two identical measurement setups in parallel, typically requiring low noise reference oscillators, phase detectors, spectrum analyzers and cross-correlation software. This method excels at ultra-low phase noise measurement and is ideal for applications that require precise phase noise measurement. This method has been notoriously complex, time-consuming and expensive.
Some labs and production facilities use specialized or integrated phase noise analyzers. These analyzers combine reference sources, phase detectors, PLLs and cross-correlation functionality into a single box. These analyzers can simplify the testing process through automation, ease of use and flexibility, but are often cost-prohibitive.
See this Signal Hound White Paper for More information on the Cross-Correlation Method:
https://signalhound.com/content/whitepapers/measure-phase-noise-with-cross-correlated-channels/
Tools and Equipment Used
- Spectrum Analyzer – Measures signal power across frequencies. Using a spectrum analyzer, the noise power and carrier power are visualized.
- Phase Noise Analyzer – A specialized tool designed specifically for phase noise measurement.
- Phase Detector – Compares DUT with a reference signal to isolate added noise.
- Reference Oscillator – A low-noise oscillator used for comparison, crucial in reducing the measured noise power.
Today’s Applications of Phase Noise Measurement
Measuring and reducing phase noise has become integral to multiple industries due to its role in ensuring system stability and precision, enabling improved performance in telecommunications, aerospace, and scientific research.
1. Communication Systems
Minimizing errors caused by signal instability ensures reliable data transmission, thereby improving overall system performance and maintaining consistent communication quality. In practical terms, tools like Signal Hound’s PN400 phase noise solution are instrumental in diagnosing and resolving phase noise issues that can otherwise compromise data integrity and system functionality in applications such as 5G networks and satellite communications. In modern applications, this is particularly important for wireless technologies like 5G, where even minor phase noise can lead to significant reductions in data throughput and connectivity reliability.
2. Radar Systems
Reducing phase noise improves target detection accuracy and resolution, ensuring that even small or distant objects can be identified with clarity. Phase noise will affect the ability to discern objects at high precision, particularly in systems requiring high dynamic range and minimal interference from added noise.
3. Scientific Research
Measuring phase noise allows for greater precision in experimental setups requiring frequency stability, especially in systems using quantum computing, nuclear magnetic resonance, and laser technology for frequency control. Additionally, in optical communications, low phase noise laser systems are essential for achieving high data transmission rates over long distances with minimal signal degradation.
Future trends
- AI Integration – Leveraging machine learning for automated analysis of vast datasets collected during phase noise measurements, can enable the identification of subtle patterns and trends that would be difficult for humans to detect. For instance, AI tools can predict phase noise behavior under different operating conditions or materials, reducing the time and cost associated with manual testing.
- Portable Instruments – Increasing portability of analyzers for field measurements, such as Signal Hound’s PN400 phase noise system, are designed to provide high performance and precision while being portable enough for use in field environments. They enable engineers to conduct on-site phase noise measurements, troubleshooting, and system optimizations without the need for large, lab-based equipment. This is particularly useful in industries like telecommunications, where rapid deployment and testing of 5G networks are essential, as well as in defense applications requiring mobile radar system assessments.
- Improved Noise Floor – Advancements in hardware have led to the development of cutting-edge components capable of achieving lower noise floors and reducing high noise levels, even in challenging environments.
Key terms
Phase Noise
The frequency-domain representation of short-term signal stability, measured in dBc/Hz.
Jitter
The time-domain equivalent of phase noise, reflecting the deviation in timing.
Instrument Phase Noise
The inherent phase noise of the measurement equipment, which must be minimized for accurate readings.
Measurement Techniques
The methods used to measure phase noise, such as spectrum analysis and cross-correlation.
Spectrum Analyzer’s Role
The spectrum analyzer’s capability to visualize and measure phase noise power relative to the carrier signal.
Phase Noise is Present
The observable noise within a signal’s frequency spectrum.
Carrier Frequency
The nominal frequency of a signal around which phase noise is analyzed.
Frequency Noise
Variations in frequency contributing to phase instability.
Laser Noise
Phase noise specifically associated with laser systems, important in optical communications.
Oscillator Phase Noise
The phase noise inherent to oscillators, affecting system stability.
Measurement Results
The output data or graph indicating phase noise levels across offsets.
Oscillator
A circuit that generates a periodic waveform, typically used in RF systems.
Frequency Stability
The measure of how consistent an oscillator’s output frequency remains over time.
dBc/Hz
Decibels relative to the carrier per Hertz, the unit used to quantify phase noise.
Phase Detector
A component used in phase noise measurement to compare input and reference signals.
Spectrum Analyzer
A device for measuring the spectrum of signals, including phase noise.
Cross-Correlation
A technique to improve measurement accuracy by canceling out uncorrelated noise.
RF
Radio frequency, a range of electromagnetic frequencies often associated with communications and radar.
Frequently Asked Questions
1. What is the significance of phase noise in RF systems?
Phase noise determines the signal’s purity, impacting communication quality and radar resolution. Low phase noise ensures better system performance.
2. How is phase noise measured?
Phase noise is measured using specialized techniques and equipment, such as spectrum analyzers, phase noise analyzers, and cross-correlation methods. These methods evaluate noise power measured relative to carrier power over frequency offsets. Measurement techniques vary, with some directly observing the noise sidebands or using a phase detector to isolate phase noise. Advanced systems may factor in limitations when measuring phase noise due to phase modulation or other distortions.
Phase noise is typically measured using spectrum analyzers or dedicated phase noise analyzers, often employing cross-correlation techniques.
3. What unit is used for phase noise?
Phase noise is expressed in dBc/Hz, indicating power density relative to the carrier per Hertz.
4. Can phase noise be reduced?
Yes, using low-noise oscillators, improved shielding, and better design techniques can minimize phase noise.
5. Why is cross-correlation used in phase noise measurement?
Cross-correlation reduces uncorrelated noise, improving measurement precision. It is especially useful for analyzing DUT phase noise in low-noise environments and is considered the most accurate type of measurement for achieving reliable results in challenging conditions.