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Phase Noise Measurement Solutions

Technical Overviews

How to Measure Phase Noise

Keysight Technologies offers a wide range of phase noise measurement solutions based on a variety of test and measurement techniques. However, finding the right solution can be a challenge.

 

This document will help guide you in how to measure phase noise and help you select the solution that best fits your specific phase noise measurement requirements.

 

Phase Noise Measurement Overview

Phase noise is one of the most important figures of merit of a signal-generating device and can be a limiting factor in a mission-critical application in aerospace and defense, as well as in communications.

 

The basic concept of phase noise when making phase noise measurements involves frequency stability, which is defined as the degree to which an oscillating source produces the same frequency throughout a specific period of time. Frequency stability consists of two components: long-term and short-term.

 

Long-term stability describes the frequency variations that occur over hours, days, months, or even years. By contrast, short-term frequency stability is about changes in the nominal carrier frequency of less than a few seconds duration. The focus of this document is on short-term frequency stability.

 

While there are many technical terms to quantify phase noise, one of the most commonly adopted phase noise measurements is the "single side-band (SSB) phase noise", ((f). Mathematically, the US National Institute of Standards and Technology (NIST) defines ((f) as the ratio of the power density at an offset frequency from the carrier to the total power of the carrier signal.

 

How to Measure Phase Noise: Summary Comparison of Measurement Techniques

A variety of measurement techniques have been developed to meet various requirements for phase noise measurements. The three most widely adopted techniques are direct spectrum, phase detector, and two-channel cross-correlation. While the direct spectrum technique measures phase noise with the existence of the carrier signal, the other two remove the carrier (demodulation) before phase noise is measured.

 

Primary Phase Noise Measurement Techniques

 

Direct spectrum technique

This is the simplest and perhaps oldest technique for making phase noise measurements. The signal from the device under test (DUT) is input into a spectrum/signal analyzer tuned to the DUT frequency, directly measuring the power spectral density of the oscillator in terms of (f).

 

As the spectral density is measured with the existence of the carrier, this method can be significantly limited by the spectrum/ signal analyzer’s dynamic range.

 

Though this method may not be useful for measuring very close-in phase noise to a drifting carrier, it is convenient for qualitative quick evaluation on sources with relatively high noise. The measurement is valid if the following conditions are met:

 

-The spectrum/signal analyzer’s inherent SSB phase noise at the offset of interest must be lower than the noise of the DUT.

 

- If the phase noise measurement implementation does not differentiate amplitude (AM) noise from the phase noise (PN), like in most of the legacy spectrum/signal analyzers, then the AM noise of the DUT must be significantly below its PN (typically 10 dB will suffice).

 

The innovative Keysight X-Series phase noise measurement application differentiates the AM noise from PN and implements the AM rejection (at offset frequency below 1 MHz), effectively removing the impact of the AM noise on the PN measurement results.

 

Phase detector techniques

When determining how to measure phase noise, you may need to separate phase noise from amplitude noise. In this case, a phase detector is required. The phase detector converts the phase difference of the two input signals into a voltage at the output of the detector. When the phase difference is set to 90° (quadrature), the voltage output will be zero volts. Any phase fluctuation from quadrature will result in a voltage fluctuation at the output.

 

Several methods have been developed based on the phase detector concept. Among them, the reference source/PLL (phase-locked-loop) is one of the most widely used methods. Additionally, the phase detector technique also enables residual/additive noise for two-port devices.

 

Two-channel cross-correlation technique

This technique combines two duplicate single-channel reference sources/PLL systems and performs cross-correlation operations between the outputs of each channel.  The two-channel cross-correlation technique achieves superior measurement sensitivity without requiring exceptional performance of the hardware components. However, the measurement speed suffers when increasing the number of correlations.

 

Please read the technical overview for more details on how to measure phase noise and to see a comparison of Keysight phase noise measurement solutions.

 

Frequently Asked Questions

 

1. What is a phase noise measurement?

 

Phase noise measurement is a characterization of the short-term stability in the phase of an oscillating RF or microwave signal. It quantifies the fluctuations in the timing of the signal's phase, which can affect the performance of the device under test (DUT).

 

2. How are phase noise measurements made?

 

Specialized equipment, such as spectrum analyzers, are used for phase noise measurements. Spectrum analyzers measure the magnitude of an input signal versus frequency within the full frequency range of the instrument. Its primary use is to measure the power of the spectrum of known and unknown signals. The terms spectrum analyzer and signal analyzer are often used interchangeably. Signal analyzer is a more accurate term for modern day analyzers that provide more comprehensive signal analysis, not only in frequency-domain, but also time and modulation domains.

 

3. When determining how to measure phase noise, how do you select the right solution?

 

Start with your specific applications, select your DUTs, your measurement, requirements, and then your investment budget. The technical overview outlines the solution components available to meet these specific needs.  

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