One of the almost spiritual quests for many design engineers with any time invested in this industry is the pursuit of a truly noiseless signal source. "Noiseless," of course, is a relative term a cursory glance at technical papers from 30 or more years ago reveals oscillator phase noise that would be considered abominable by today's standards. And phase noise is just one of the noise components in evaluating a microwave source as being low noise. It must also exhibit low second- and higher-order harmonic levels and low levels of spurious signal products. The quest for a truly noiseless microwave source has led to many truly creative frequency synthesis approaches over the years, based on such techniques as temperature control and compensations, phase-locked loops (PLLs) and other frequency-stabilizing or frequency-comparing techniques, and even digital correction techniques.
Credit is due to the relentless designers who never stop seeking improved microwave source noise levels. Several examples can be found in the upcoming Cover Feature of the May 2011 issue of Microwaves & RF, as well as in work detailed by authors from Stanford Research Systems. Not only is the task of developing an oscillator or synthesis architecture that can reduce phase noise difficult, but the very job of measuring phase noise is a challenge, requiring peerlessly calibrated and maintained test sets. These efforts, to achieve lower levels of phase noise at RF and microwave frequencies, are making possible the higher orders of digital modulation that are now enabling higher data rates over wireless transmissions. So, when you chance across some engineers at the upcoming IMS show in Baltimore next month, if they are claiming lower phase noise for an oscillator or synthesizer, tell them "thank you."