Agilent Understanding Phase Noise Needs and Choices in Signal Generation 5991-1744EN [12] free download

File name Understanding Phase Noise Needs and Choices in Signal Generation 5991-1744EN [12].pdf

Understanding Phase Noise Needs and Choices in Signal Generation Application Note The phase noise performance of an RF or microwave signal generator is often a key factor in determining how well it fits an application. For example, phase noise performance is especially important in the testing of high-performance systems such as Doppler radars and cognitive or software-defined radios (SDRs). Excellent phase noise specifications are also important when using a signal generator for oscillator substitution or analog-to-digital converter (ADC) testing. When evaluating an RF or microwave signal generator for use in such applications, several performance factors are worth a close look: spurious, harmonics, broadband noise, AM noise and phase noise. Looking specifically at phase noise performance, it is affected by the internal architecture of the instrument and the features and capabilities layered on top of that architecture. The most common architectures are single-loop and double-loop, and these will be explored later in this note. Available features include digital modulation capabilities, pulse capabilities and multi-unit synchronization--and the presence of these can affect phase noise performance. In both the design and evaluation of a signal generator, phase noise performance includes tradeoffs such as cost, switching speed and optimization at various frequency offsets from the carrier signal. To address a variety of requirements, some signal generators offer two or more levels of phase noise performance (e.g., standard and optional capabilities). Others allow optimization of phase noise performance at wide or narrow offsets. Still others may allow the user to selectively degrade phase noise performance and observe the effects on the device-under-test (DUT). To provide context, this note first discusses the fundamentals of phase noise before moving on to a closer look at architectural choices and the effects of various functionality alternatives. The discussion then moves on to an overview of Agilent signal generators and the built-in capabilities they offer to enhance or selectively degrade phase noise performance. The foundation: stability and noise Any discussion of phase noise is mostly concerned with the frequency stability of a signal. Long-term stability, perhaps of an oscillator, may be characterized in terms of hours, days, months, or even years. Short-term stability refers to frequency changes that occur over a period of a few seconds or less. These short-cycle variations have a much greater effect on systems that rely on extreme pr