Editing Spread spectrum (section)

== Clock signal generation ==
{{more citations needed section|date=January 2020}}
[[Image:Aaronia Spectrum Analyzer Software.jpg|thumb|Spread spectrum of a modern switching power supply (heating up period) incl. waterfall diagram over a few minutes. Recorded with a NF-5030 EMC-Analyzer]]
Spread-spectrum clock generation (SSCG) is used in some [[synchronous circuit|synchronous digital systems]], especially those containing microprocessors, to reduce the spectral density of the [[electromagnetic interference]] (EMI) that these systems generate.  A synchronous digital system is one that is driven by a [[clock signal]] and, because of its periodic nature, has an unavoidably narrow frequency spectrum.  In fact, a perfect clock signal would have all its energy concentrated at a single frequency (the desired clock frequency) and its harmonics.

=== Background ===
Practical synchronous digital systems radiate electromagnetic energy on a number of narrow bands spread on the clock frequency and its harmonics, resulting in a frequency spectrum that, at certain frequencies, can exceed the regulatory limits for electromagnetic interference (e.g. those of the [[Federal Communications Commission|FCC]] in the United States, [[JEITA]] in Japan and the [[International Electrotechnical Commission|IEC]] in Europe).

Spread-spectrum clocking avoids this problem by reducing the peak radiated energy and, therefore, its electromagnetic emissions and so comply with [[electromagnetic compatibility]] (EMC) regulations. It has become a popular technique to gain regulatory approval because it requires only simple equipment modification. It is even more popular in portable electronics devices because of faster clock speeds and increasing integration of high-resolution LCD displays into ever  smaller devices. As these devices are designed to be lightweight and inexpensive, traditional passive, electronic measures to reduce EMI, such as capacitors or metal shielding, are not viable. [[Active EMI reduction]] techniques such as spread-spectrum clocking are needed in these cases.

=== Method ===
In PCIe, USB 3.0, and SATA systems, the most common technique is downspreading, via [[frequency modulation]] with a lower-frequency source.<ref>{{cite web |title=Spread Spectrum Clocking |url=https://www.microsemi.com/document-portal/doc_download/135439-white-paper-spread-spectrum-clocking |website=Microsemi}}</ref> Spread-spectrum clocking, like other kinds of [[dynamic frequency change]], can also create challenges for designers. Principal among these is clock/data misalignment, or [[clock skew]]. A [[phase-locked loop]] on the receiving side needs a high enough bandwidth to correctly track a spread-spectrum clock.<ref name="IM2013">{{cite web |author1=Item Media |title=Spread Spectrum Clock Generation – Theory and Debate |url=https://interferencetechnology.com/spread-spectrum-clock-generation-theory-and-debate/ |website=Interference Technology |date=19 March 2013}}</ref>

Even though SSC compatibility is mandatory on SATA receivers,<ref>{{cite web |title=CATC SATracer / Trainer Application Note: Spread Spectrum Clocking |url=https://cdn.teledynelecroy.com/files/appnotes/satracer_ssc_appnote.pdf |website=CATC |access-date=20 May 2023 |date=July 2, 2003}}</ref> it is not uncommon to find expander chips having problems dealing with such a clock. Consequently, an ability to disable spread-spectrum clocking in computer systems is considered useful.<ref>[http://www.thomas-krenn.com/de/wiki/Raid_Controller_erkennt_Western_Digital_Raid_Edition_III_HDDs_nicht Western Digital Raid Edition III HDDs werden vom RAID Controller nicht erkannt] (Thomas Krenn Wiki)</ref><ref>[http://www.intel.com/cd/channel/reseller/emea/deu/support/lcs/faq/334571.htm Intel Speichersystem SS4000-E: Festplatten, wie beispielsweise die Western Digital WD2500JS SATA, werden nicht erkannt. Woran liegt das?] (Intel Reseller-Center)</ref><ref>{{Webarchive |url=https://web.archive.org/web/20100429194819/http://seagate.custkb.com/seagate/crm/selfservice/search.jsp?DocId=214011&NewLang=en |title=SSC Toggle Utility – Barracuda 7200.9 |date=2010-04-29}} (Seagate Knowledge Base)</ref>

=== Effect ===
Note that this method does not reduce total [[radiation|radiated]] energy, and therefore systems are not necessarily less likely to cause interference. Spreading energy over a larger bandwidth effectively reduces electrical and magnetic readings within narrow bandwidths. Typical [[measuring receiver]]s used by EMC testing laboratories divide the electromagnetic spectrum into frequency bands approximately 120&nbsp;kHz wide.<ref>American National Standard for Electromagnetic Noise and Field Strength Instrumentation, 10 Hz to 40 GHz—Specifications, ANSI C63.2-1996, Section 8.2 Overall Bandwidth</ref> If the system under test were to radiate all its energy in a narrow bandwidth, it would register a large peak. Distributing this same energy into a larger bandwidth prevents systems from putting enough energy into any one narrowband to exceed the statutory limits. The usefulness of this method as a means to reduce real-life interference problems is often debated,<ref name=IM2013/> as it is perceived that spread-spectrum clocking hides rather than resolves higher radiated energy issues by simple exploitation of loopholes in EMC legislation or certification procedures. This situation results in electronic equipment sensitive to narrow bandwidth(s) experiencing much less interference, while those with broadband sensitivity, or even operated at other higher frequencies (such as a radio receiver tuned to a different station), will experience more interference.

FCC certification testing is often completed with the spread-spectrum function enabled in order to reduce the measured emissions to within acceptable legal limits.  However, the spread-spectrum functionality may be disabled by the user in some cases. As an example, in the area of personal computers, some [[BIOS]] writers include the ability to disable spread-spectrum clock generation as a user setting, thereby defeating the object of the EMI regulations. This might be considered a [[loophole]], but is generally overlooked as long as spread-spectrum is enabled by default.