Phase noise measurement using RF spectrum analyzer

Phase noise measurement is crucial in characterizing the performance of RF (Radio Frequency) oscillators and signal sources. Phase noise refers to the short-term fluctuations in the phase of a signal, typically expressed in units of dBc/Hz (decibels relative to the carrier per Hz bandwidth). An RF spectrum analyzer is commonly used to measure phase noise. Let us understand Phase noise measurement using RF spectrum analyzer. Explore phase noise test set up used for its measurement with a spectrum analyzer.

As we know phase noise is the measurement which helps in verifying the purity of the RF signal under test. It is measured for various RF components, devices and systems which include crystal oscillator, Local Oscillator, Synthesizer, Up/Down Frequency converters etc.
Refer RF phase noise and phase jitter basics and conversion➤.

Phase Noise is specified as follows:
"Phase Noise is equal to -60dBc/Hz at 1KHz" for say RF synthesizer output of say 6 GHz.

How to measure phase noise with a spectrum analyzer

Here's an explanation of the test setup and process.
Test Setup:
➨RF Signal Source: The first component of the setup is the RF signal source, which is the device under test (DUT). This could be a signal generator, a frequency synthesizer, an oscillator, or any other RF device that generates a signal. The signal source should have stable frequency and amplitude characteristics.
➨Frequency Reference: To accurately measure phase noise, a stable frequency reference is necessary. This reference ensures that the analyzer and the DUT are both operating at the same frequency. A high-quality frequency reference, such as a precision oscillator or an atomic clock, is typically used for this purpose.
➨RF Spectrum Analyzer: The key instrument in the setup is the RF spectrum analyzer. This device is capable of measuring the amplitude of signals across a range of frequencies. It provides a graphical representation of the signal spectrum, showing the amplitude (power) of the signal as a function of frequency.
➨Phase Noise Measurement Software: Some modern spectrum analyzers come with specialized phase noise measurement software. This software is designed to accurately analyze and quantify phase noise characteristics from the spectrum analyzer's measurements.

The figure-1 depicts typical setup used for RF phase noise measurement. As shown in the figure, RF output of DUT (Device Under Test) say RF synthesizer is connected with RF INPUT of Spectrum Analyzer.

Following settings need to be done in the spectrum analyzer to measure phase noise with optimum accuracy for the above specification.
➨Center Frequency(Fc) = 6 GHz
➨Span = 10 KHz
➨ Marker (δ) = 1KHz
➨RBW (Resolution Bandwidth) = 100 Hz
➨VBW (Video Bandwidth) = 1 KHz (Check the display and set accordingly to obtain desired result).

As shown in the figure, RBW has great effect on RF phase noise measurement using Spectrum Analyzer. Hence the optimum RBW is selected for the best measurement corresponding to RF output and span.

Phase Noise Test Procedure

Following steps are followed for phase noise measurement.
1. Configure signal source and spectrum analyzer to operate at desired frequency as mentioned in example above.
2. Calibrate spectrum analyzer to measure signal amplitude accurately. Do appropriate settings of RBW and VBW as mentioned in example above.
3. Connect the output of the RF signal source to the input of the spectrum analyzer.
4. Set the spectrum analyzer to display a narrow frequency span around the carrier frequency of the signal.
5. Enable the phase noise measurement function on the spectrum analyzer or use dedicated phase noise measurement software.
6. Initiate the measurement and allow the spectrum analyzer to capture and analyze the phase noise characteristics of the signal.
7. The spectrum analyzer will display the phase noise profile of the signal, typically in units of dBc/Hz.

Conclusion : In summary, phase noise measurement using an RF spectrum analyzer involves configuring the test setup with an RF signal source, a frequency reference, and the spectrum analyzer, followed by performing the measurement, analysis, and optimization of phase noise characteristics to ensure the desired performance of RF systems and components.

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