Measuring Phase Noise

Dear Editor,
I read with great interest your report on phase noise in the February issue of Microwaves & RF. For the most part, the article made valid points about the vagaries of phase-noise specifications in product data sheets. The phase noise decreases with distance from the carrier frequency. In some systems, close-in phase-noise performance is not critical and the further-out phase noise, or the noise floor, can play a greater role in determining the overall sensitivity and accuracy of the system, such as in direction-finding (DF) radar systems. Close-in phase noise is critical for many modern communications systems—especially those based on digital modulation schemes and the use of vector (amplitude and phase) quantities to achieve high data rates and high spectral efficiency with limited channel bandwidths.

Although the article did an adequate job of reviewing available low-phasenoise oscillators on the market, it did a poor job of education in the area of phase-noise measurements. The one mention of the outdated HP 3048 phase-noise test set from Hewlett- Packard (now Agilent Technologies) hardly did justice to the handful of high-performance measurement solutions available from several test-equipment manufacturers, including Aeroflex, Agilent, and Rohde & Schwarz. In particular, the Agilent and Rohde & Schwarz phasenoise test sets offer outstanding performance for low-phase-noise measurements, albeit with a higher price tag than a high-performance spectrum analyzer. Admittedly, the spectrum analyzer can do other things, but the dedicated phasenoise test sets can provide unmatched accuracy and repeatability.

Regards,
Olivia Dunham
Communications Manager
Configuration Analysis Management
Danbury, CT

Editor’s Note:
Our apologies to Ms. Dunham for being misleading in any way in the phase-noise report. First of all, the story was meant to provide a brief overview and not an all-encompassing survey of low-noise sources. Second, regarding the test equipment, the HP 3048 was offered as a historical perspective and an example, and not as the definitive solution to measuring phase noise. As Ms. Dunham suggests, readers are invited to visit the Aeroflex, Agilent, and Rohde & Schwarz web sites for more information on their phase-noise measurement systems.