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Agilent E5052B Signal Source Analyzer
Boosting PLL Design Efficiency
From free-running VCO characterizations
to closed-loop PLL evaluations
Application Note

Table of Contents
1. PLL Synthesizer Basics .....................................................................................................................2
2. VCO Characterization ........................................................................................................................3
2-1. VCO characteristic parameters to be measured ...........................................................3
2-2. How to tame a free-running VCO ......................................................................................4
2-3. Tuning characteristics..........................................................................................................6
2-4. Oscillator pushing characteristics ....................................................................................8
2-5. Low noise DC Sources are required for precise VCO characteristics ...................10
2-6. Load-pulling characteristics .............................................................................................13
2-7. Harmonics and spurs ..........................................................................................................17
2-8. Tuning delay and frequency setting time......................................................................17
3. Frequency Divider Evaluation ........................................................................................................19
4. Total PLL Performance Test...........................................................................................................21
4-1. General topics on PLL design...........................................................................................21
4-2. Fundamental parameters of PLL frequency synthesizer performance ................ 22
4-3. Frequency/power/phase transient measurements with the E5052B ................. 23
4-4. "Cut & Try" optimization for loop filter design ............................................................27
Summary..................................................................................................................................................29
Literature References ..........................................................................................................................29
References ..............................................................................................................................................29

Introduction
A phase-locked loop (PLL) technique is widely used in today's advanced
communications and broadcasting systems, and PLL frequency synthesizers
are an indispensable part of the system. Recent advances of higher data rates
and more channels per unit of bandwidth have accelerated the need for higher
performance PLL frequency synthesizers.

The requirements of modern PLL synthesizers are becoming stricter in terms
of frequency stability, frequency switching speed, phase noise and reliability
as well as in size, weight and power consumption. These constraints make
PLL synthesizer design more challenging and time-consuming.

This application note tells you how to design and evaluate PLL synthesizers
more efficiently. Examples also show how related PLL system components such
as voltage controlled oscillators (VCOs), reference oscillators and frequency
dividers/prescalers are characterized and evaluated.
1. PLL Synthesizer Basics A typical PLL frequency synthesizer consists of several key components, shown
in Figure 1. Although pure digital PLLs have become popular in recent compact
systems, analog VCOs and loop filters are still commonly used in many signal
sources. Evaluating VCO performance is the first step toward designing a better
PLL synthesizer. The second step is to design the optimal loop filter for lower
phase/spurious noise and faster frequency transient response.

Signal output
ref
Frequency M out
Phase
divider comparator Loop filter VCO
(1/M)
out = K