For years now, government institutions have been regulating the amount of EMI (electromagnetic interference) an electronic device or system can emit. Their efforts primarily target lowering dissipated power and eliminating any interference to the function of other surrounding devices as a result of EMI. Spread-spectrum clocking is a popular implementation for reducing EMI in synchronous systems.
EMI is the energy resulting from a periodic source in which most of the energy becomes a single fundamental frequency. The influence of these unwanted signals can manifest itself in the limited operation of other devices and systems. In some cases, the EMI-generated disturbance can make it impossible for these devices or systems to operate. Because an electromagnetic signal must have a source, synchronous systems are ideal candidates for generating excessive EMI. Within a system, the coupling paths in PCBs (printed-circuit boards) transmit the generated EMI that affects other system components. However, EMI can occur even in the absence of a conductive medium, such as an electric conductor or dielectric. In most cases, EMI results from a combination of conduction and radiation.
The primary PCIe (PCI Express) model implements a synchronous-clocking scheme. That is, the same 100-MHz clock source generates the reference clock for PCIe devices. Furthermore, in the case of a motherboard, the traces on the PCB can act as coupling paths to facilitate the transmission of EMI to the surrounding devices. The disturbance that occurs can affect not only the system but also other surrounding systems when EMI travels through the atmosphere in the form of radiation.
One method of minimizing the EMI that a device generates is to keep the disturbing signals below a certain level. You accomplish this goal by modulating the disturbing signals across a wider frequency range, thus spreading the energy across a range of frequencies rather than concentrating it at one frequency. In PCIe systems, the modulation of the reference clock is spread-spectrum clocking.
The most common modulation techniques are center-spread and down-spread. The center-spread approach applies the modulated signal in such a way that the nominal frequency sits in the center of the modulated frequency range. That is, half of the modulated signals deviate above the nominal frequency, and the other half deviate below it. A down-spread approach also results in a range of deviated frequencies. However, in the down-spread approach, the modulated signals deviate below the nominal frequency.
Spread-spectrum clocking reduces EMI in embedded systems figure 1Many PCIe systems implement EMI-minimizing spread-spectrum clocking by spreading the spectral energy of the clock signal over a wide frequency band. In spread-spectrum-clocking systems, PCIe components generally must use a reference clock from the same source. This approach allows a transmitter PLL (phase-locked-loop) and a receiver-clock-recovery function, or clock-data-recovery circuit, to track the modulation frequency and remain synchronous with each other. If only one side of the link uses a spread-spectrum-clocking reference clock, the transmitter and receiver circuits cannot properly track one another. For example, if a PCIe add-in card interfaces to a spread-spectrum-clocking system and also implements a cable connection to a downstream card that is using a constant-frequency-clock source, the downstream interface will be unable to connect.
The PCIe base specification provides guidelines for modulating the reference-clock input to PCIe devices. At a high level, the PCIe specification uses the down-spread approach when using a 30- to 33-kHz-wave signal as the modulating frequency to the 100-MHz clock, resulting in a frequency range of 99.5 to 100 MHz (Figure 1).
I think there was a great deal of discussions on this conflict on the same topic about the benefits of spread spectrum clocking earlier :)