Meaning
Electromagnetic interference reduction techniques modulate the frequency of high-speed system clocks to distribute the radiated energy over a wider bandwidth. Digital hardware designers implement spread spectrum clocking to lower the peak electromagnetic emissions from microprocessors, memory interfaces and high-speed serial buses. This modulation helps the product pass strict regulatory emissions limits without heavy shielding.
Frequency Modulation
The clock generator modulates the operating frequency with a specific waveform, such as a triangle wave, causing the frequency to sweep across a small range. Operating with spread spectrum clocking replaces a high-power spike at a single frequency with a wider, lower-power emission pattern that is much less likely to interfere with sensitive analog radio receivers or wireless communication links located on the same circuit board.
Compliance Testing
Standard tests measure the peak power of emissions across specific frequency bands to verify that they do not exceed legal limits. Utilizing spread spectrum clocking reduces the measured peak emissions by several decibels, allowing the device to meet FCC and CE requirements without requiring expensive metal enclosures.
Jitter Consideration
Modulating the clock frequency introduces cycle-to-cycle timing variations that can affect high-speed communications. Because spread spectrum clocking increases jitter, it must be disabled or carefully tuned on interfaces that require tight timing sync to prevent data transmission errors.
