Dual Fractional-N Synthesizers/PLLs -- Frequently Asked Questions

By Skyworks Solutions, Inc.

This article is a follow-up to the Skyworks' article Basics of Dual Fractional-N Synthesizers/PLLs that provides a general discussion of the use of fractional-N synthesizers in a low-cost, low-power radio. The article provides answers to a series of frequently asked questions (FAQs) related to fractional-N and their respective evaluation boards.

General questions covered include:

• What are the design trade-offs associated with fractional-N synthesizers?
• What is the primary benefit of fractional-N synthesizers?
• Can I use a lower reference frequency with fractional-N synthesizers?
• What spurs can be expected from fractional-N synthesizers versus integer-N synthesizers?
• What is meant by the term "spur-free"?
• Does fractional-N offer improved phase noise over integer-N?
• Does fractional-N offer improved switching time over integer-N?

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•Application Note: Dual Fractional-N Synthesizers/PLLs -- Frequently Asked Questions