Spread Spectrum Clocking
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Spread Spectrum Clocking
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From Wikipedia, the free encyclopedia
Spreading the frequency domain of a signal
This section needs additional citations for verification . Please help improve this article by adding citations to reliable sources . Unsourced material may be challenged and removed. ( January 2020 ) ( Learn how and when to remove this template message )
^ Torrieri, Don (2018). Principles of Spread-Spectrum Communication Systems, 4th ed .
^ Jump up to: a b David Kahn, How I Discovered World War II's Greatest Spy and Other Stories of Intelligence and Code, CRC Press - 2014, pages 157-158
^ Tony Rothman, Random Paths to Frequency Hopping, American Scientist, January–February 2019
Volume 107, Number 1, Page 46 americanscientist.org
^ Jonathan Adolf Wilhelm Zenneck, Wireless Telegraphy, McGraw-Hill Book Company, Incorporated, 1915, page 331
^ Denis Winter, Haig's Command - A Reassessment
^ Danilewicz later recalled: "In 1929, we proposed to the General Staff a device of my design for secret radio telegraphy which fortunately did not win acceptance, as it was a truly barbaric idea consisting in constant changes of transmitter frequency. The commission did, however, see fit to grant me 5,000 złotych for executing a model and as encouragement to further work." Cited in Władysław Kozaczuk , Enigma: How the German Machine Cipher Was Broken, and How It Was Read by the Allies in World War II , 1984, p. 27.
^ Ari Ben-Menahem, Historical Encyclopedia of Natural and Mathematical Sciences, Volume 1, Springer Science & Business Media - 2009, pages 4527-4530
^ 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
In telecommunication and radio communication , spread-spectrum techniques are methods by which a signal (e.g., an electrical, electromagnetic, or acoustic signal) generated with a particular bandwidth is deliberately spread in the frequency domain , resulting in a signal with a wider bandwidth . These techniques are used for a variety of reasons, including the establishment of secure communications, increasing resistance to natural interference , noise , and jamming , to prevent detection, to limit power flux density (e.g., in satellite downlinks ), and to enable multiple-access communications.
Spread spectrum generally makes use of a sequential noise -like signal structure to spread the normally narrowband information signal over a relatively wideband (radio) band of frequencies. The receiver correlates the received signals to retrieve the original information signal. Originally there were two motivations: either to resist enemy efforts to jam the communications (anti-jam, or AJ), or to hide the fact that communication was even taking place, sometimes called low probability of intercept (LPI). [1]
Frequency-hopping spread spectrum (FHSS), direct-sequence spread spectrum (DSSS), time-hopping spread spectrum (THSS), chirp spread spectrum (CSS), and combinations of these techniques are forms of spread spectrum. The first two of these techniques employ pseudorandom number sequences—created using pseudorandom number generators —to determine and control the spreading pattern of the signal across the allocated bandwidth. Wireless standard IEEE 802.11 uses either FHSS or DSSS in its radio interface.
The idea of trying to protect and avoid interference in radio transmissions dates back to the beginning of radio wave signaling. In 1899, Guglielmo Marconi experimented with frequency-selective reception in an attempt to minimize interference. [2] The concept of Frequency-hopping was adopted by the German radio company Telefunken and also described in part of a 1903 US patent by Nikola Tesla . [3] [4] Radio pioneer Jonathan Zenneck 's 1908 German book Wireless Telegraphy describes the process and notes that Telefunken was using it previously. [2] It saw limited use by the German military in World War I , [5] was put forward by Polish engineer Leonard Danilewicz in 1929, [6] showed up in a patent in the 1930s by Willem Broertjes ( U.S. Patent 1,869,659 , issued Aug. 2, 1932), and in the top-secret US Army Signal Corps World War II communications system named SIGSALY .
During World War II, Golden Age of Hollywood actress Hedy Lamarr and avant-garde composer George Antheil developed an intended jamming-resistant radio guidance system for use in Allied torpedoes , patenting the device under U.S. Patent 2,292,387 "Secret Communications System" on August 11, 1942. Their approach was unique in that frequency coordination was done with paper player piano rolls - a novel approach which was never put into practice. [7]
Spread-spectrum clock generation (SSCG) is used in some 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. 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 FCC in the United States, JEITA in Japan and the IEC in Europe).
Spread-spectrum clocking avoids this problem by using one of the methods previously described to reduce 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.
However, 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 . Consequently, an ability to disable spread-spectrum clocking in computer systems is considered useful.
Note that this method does not reduce total 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 receivers used by EMC testing laboratories divide the electromagnetic spectrum into frequency bands approximately 120 kHz wide. [8] 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, 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.
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We're here to settle, once and for all, the pros and cons of spread-spectrum clocking as an EMI countermeasure.
In case you're unaware, many systems today employ frequency-modulation of their clock signals wherever possible. This spreads the energy from narrow spectral lines into wider bands, reducing peak signal strength, and therefore making it easier to come in below regulatory limits.
Spectra for different modulation indices. (Source: Agilent)
This has been the cause of debate and disagreement in the industry for 15 years or more – probably for as long as chips have been available to easily implement modulated clocks. Yet lucid, balanced arguments seem non-existent. In an attempt to shed some light on the issue, four experienced engineers, me included, will present their thoughts below. Call it a debate.
Glen Chenier I've been through many bouts of measuring UUTs' radiated RF emissions. In most cases they failed, but the emitted frequencies plot was very handy to have back in my own lab to rectify the problems . Containing a signal is not so hard with proper PCB layout, ground planes, bypassing, shielding, slowing digital risetimes, and all the other tricks that need to be learned. Sometimes they work both ways, making your product immune to outside sources of RF too.
My products were for telecommunications, so the clock-modulation option to sneak excessive emission levels under the FCC carpet was not an option. Accumulated timing jitter is bad enough without adding more to the clocks. The only reason FM appears to work is that the offending signal sweeps through the narrow detector bandwidth only a fraction of the total time, reducing the apparent energy level. In a bandwidth wider than the peak-to-peak sweep frequencies (such as a television receiver or an adjacent analog trace such as a laser modulator), that signal is seen at full strength.
Spreading RF emissions is like stepping in a cowpie – it doesn't disappear, it just gets spread around.
Jason Frels All of the energy is still there and being radiated, but in many cases spreading it out reduces interfere
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