Spread spectrum

[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.

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.

Typical measuring receivers used by EMC testing laboratories divide the electromagnetic spectrum into frequency bands approximately 120 kHz wide.

The usefulness of this method as a means to reduce real-life interference problems is often debated,[9] 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.

FCC certification testing is often completed with the spread-spectrum function enabled in order to reduce the measured emissions to within acceptable legal limits.

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.