PLL Skript

Spurious Sidebands and Phase Noise in Frequency Synthesizers

Christian Münker

Spectral purity of frequency synthesizers in communication circuits has become more and more important due to increasing requirements on spectral leakage in the transmit path and sensitivity in the RX path in crowded frequency bands. At the same time, these requirements have to be fulfilled with less expensive components in a shorter design cycle to be competitive. However, phase locked loops are complex building blocks that are hard to analyze in a strictly mathematical way and hard to simulate. This tutorial tries to bridge the gap between precise computer simulations giving little insight into system mechanisms and between hand-waving simplifications allowing only qualitative answers based on questionable assumptions. Currently, there is little material available covering the aspects of spurious sidebands and phase noise generation in PLLs in depth while still keeping an engineering point of view, i.e. reducing complexity to achieve an understanding of the system that allows making the right design choices in a short time.

Part I reviews some PLL basics needed for the following analyses: chapter 2 takes a walk round the loop and looks at PLLs from a control theory point of view, with the focus on different loop filter architectures and their influence on settling time etc. A novel approach is presented for approximating the influence of the loop filter on spurious and phase noise performance that will be used throughout this tutorial. Chapter 3 covers the building blocks of a PLL like phase detector, VCO etc. and their influence on system behavior.

Part II looks at the mechanisms of spurious sideband generation: chapter 4 starts with a little modulation theory to explain how low frequency disturbances in the system can create spurious sidebands in the radio frequency domain. Some typical examples of real-life disturbances are analyzed, leading to simple equations that allow making intelligent design decisions. Chapter 5 gives an overview of different disturbance mechanisms in PLLs, going a little further into the details of practical PLL design. In chapter 6, the specialties of fractional-N PLLs concerning spurious generation are analyzed.

Part III deals with noise in PLLs: After an introduction to the concepts of phase noise and jitter in chapter 7, the next chapter covers the noise mechanisms in different blocks and their contribution to the overall PLL noise.

Finally, Part IV is as a kind of toolbox with some of the mathematical tools needed for PLL analysis: Fourier analysis, noise and modulation theory etc. are explained in greater detail than in the main parts.

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