Grasp Pulsed Phase Noise Measurements

June 8, 2016
This application note describes pulsed phase noise in detail, explaining how it differs from continuous-wave (CW) phase noise along with techniques to perform measurements.

Phase noise is a key parameter that can limit system performance. With the development of new and more advanced RF systems, phase noise of oscillators and transmitters cannot be overlooked. In the application note, "Pulsed Phase Noise Measurements," Rohde & Schwarz discusses phase noise measurements of pulsed RF carriers for applications like radar systems.

The application note begins by providing a brief introduction to radar, explaining that most radar systems employ pulse modulation. Some radar velocity measurement examples are presented to demonstrate the importance of phase noise in radar systems. The characteristics of pulsed waveforms are then explained in detail. Important parameters are discussed, including pulse width, pulse repetition interval (PRI), pulse repetition frequency (PRF), and duty cycle.

Phase noise of pulsed carriers is then closely examined. As an example, the spectrum of a 1-GHz carrier that is modulated with a 10-μs wide pulse that has a PRF of 10 kHz is presented. The document next describes the differences between pulsed and continuous-wave (CW) phase noise. A plot of both the CW and pulsed phase noise of a 1-GHz carrier is presented, followed by an explanation of why they differ from one another.

To demonstrate the effect that pulse parameters have on phase noise performance, three different phase noise measurements are presented. Each measurement was made with a constant PRF of 10 kHz, but with a different pulse width. The first measurement was made using a 10-μs pulse width, which was increased to 50 μs for the second measurement. Increasing the pulse width from 10 to 50 μs resulted in improved phase noise performance. The third measurement was made with a 1-μs pulse width. This measurement demonstrated that decreasing the pulse width from 10 to 1 μs worsened the phase noise.

These measurements proved that one should not expect an oscillator’s pulsed-carrier phase noise to be the same as its CW phase noise. The application note concludes with a description of the Rohde & Schwarz FSWP phase noise analyzer, comparing this measurement solution to traditional test methods.

Rohde & Schwarz, Mühldorfstrasse 15, 81671 München, Germany; 800-837-8772

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