Spread spectrum clock reduces EMI for hard disks

Spread Spectrum Clock Generation (SSCG) is an efficient approach to reducing electro-magnetic interference emissions in many high-speed digital systems including hard disk drives.

The most widely used conventional techniques to control electro-magnetic interference (EMI) emissions are shielding and filtering by using passive components. These techniques become difficult to employ when electronic systems become faster, more complex and portable.

Shielding is the least desirable method of EMI reduction in most systems today, especially for portable and handheld products. That's because it increases the size, weight and cost of the system. Additionally, the increase in labor costs could be substantial, since shielding these products is difficult to automate in the manufacturing process.

As for filtering, this technique is not systemic. Since designers cannot measure EMI emissions until after the system is built, they are forced to provide filter placement in many suspect locations and waste valuable time and PCB space.

By contrast, SSCG offers good system-wide coverage as well as programmability, it does not degrade timing signal quality and it can reduce the number of pc-board layers and overall board space needed for a design.

SSCG reduces the radiated emissions of the digital clock and timing signals by frequency modulating the system clock with a low frequency signal. This creates a frequency spectrum with sideband harmonics. Since the narrowband repetitive system clock is intentionally broadbanded, the peak spectral energy contained in the fundamental and harmonic frequencies is simultaneously reduced.

All clock and timing signals derived from the spread spectrum clock are modulated at the same percentage amount. This leads to a dramatic EMI reduction throughout the system.

During electro-magnetic compliance testing, frequency modulation can be adjusted by the programmable digital inputs. SSCG also allows additional programmable EMI reduction and timing functions to be integrated into the same product.

In the system, set-up and hold times are referenced only to the rising edge of the timing signal. Since the rise and fall time is not changed when spread spectrum is used, the critical set-up and hold time margins are still maintained, preserving timing signal quality.

Instead of maintaining a constant frequency, the SSCG technique modulates the system clock frequency with a much smaller frequency — typically in the range of 30 to 90 kHz — to control and reduce EMI emissions at its source, the system clock.

In a reference application of SSCG to a hard disk drive, the input clock to main system processor/controller ASIC is modulated by about +/-1.75 percent. The measured EMI levels are reduced significantly, greater than 10dB for many harmonics, resulting in electro-magnetic compliance for the system.

The measured relative field strength of the fifth harmonic can be reduced by more than 10dB using +/-1.75 percent center spread modulation. Much higher reductions approaching 20dB can be obtained by using higher spread percent and higher profile frequency modulation in the range of 60 to 120 kHz.

An additional non-spread clock might be used in the other parts of the system where a stable non-modulated clock is required. The difficulty in meeting the required emission levels is further increased when combining multiple hard drives in a single chassis, such as in work stations, since emissions produced by each drive is additive. EMI radiation can be easily controlled by programming the spread percent value up to +/-5.0 percent, depending of the system emission levels.

In the case of hard disk drives used in desktop and notebook PCs, the spread spectrum technique reduce the number of board layers from six or four to two. In this case, the additional EMI reduction obtained by the use of Spread Spectrum Clock (SSC) eliminates the need for additional power supply and ground layers, resulting in only two-layer board, thus further reducing the system cost.