EMI CONTROL

Reducing EMI with Low-EMI Clock Oscillators

Tom Tritthart

Spread-spectrum technology allows common devices to significantly reduce interference.

Spread-spectrum technology can be used to reduce electromagnetic interference (EMI) when integrated into common 8- and 14-pin dual-inline packages and surface-mount versions as small as 5 x 7 mm. Such drop-in replacement clock oscillators are ideal when EMI needs to be reduced by as much as 20 dB.

Typical system designs start out with a fundamental clock source, such as a crystal, ceramic, or can oscillator, at some frequency; say 48 MHz, which drives a processor, memory, serial and USB interfaces, and some front panel controls. The 48-MHz clock is converted into other timing components such as clock multipliers found in phase-locked loop (PLL)-based processors, memory controllers, and interface controllers. The original 48-MHz clock quickly becomes 96 MHz or 24 MHz to drive a serial interface. These fundamental and harmonic frequencies can make it very difficult for compliance engineers to achieve certification.

During the design phase, choosing the proper package and frequency for the clock source increases flexibility in future modifications. Even after the design phase is done, there are always hot spots that require some form of filtering or shielding. This is sufficient if all that is needed is to slow down the rise time of one or two specific nets. However, this is not usually the case. When an EMC engineer sees that agency limits have been exceeded by 10 dB in several higher-frequency harmonics, production must wait until compliance is achieved. When this happens, there are three possible ways to solve the problem: redesign in an attempt is reduce EMI, filter and shield every net that is offending compliance, or change the bill of material to call out a low-EMI clock oscillator.

Low-EMI clock oscillators, (ASSM, ACSH, ACSO, and ASSL), operate over a frequency range of 4 to 128 MHz at 3.3 and 5.0 V in commercial and industrial temperature ranges. Products cover a wide range of applications, including automotive, medical, industrial control, and computer peripherals such as printers, scanners, copiers, and any other digital systems requiring EMI clocking solutions.

Spread-spectrum clocking (SSC) takes advantage of the fact that a frequency-modulated carrier will have lower peak energy than a nonmodulated carrier. By modulating the carrier frequency, the energy is spread out over a wider range of frequencies, thereby reducing the peak energy contained in any one frequency. When comparing a modulated clock to a nonmodulated clock on a spectrum analyzer, it can be seen that the peaks of the modulated clock and its harmonic frequencies are lower in relative strength.

The difference in relative strength of the energy of the clock is measured in decibels. Sweeping the frequency of the fundamental clock back and forth at some rate causes a reduction in peak energy: the wider the spread, the greater the peak-energy reduction. To determine how much spread is needed for a given application, use the simple formula below to calculate the necessary decibel reduction. This formula assumes an ideal clock with a 50% duty cycle and predicts only the EMI reduction of odd harmonics. The calculation for dB reduction

where

F = frequency in MHz, and
BW = total % spread (2.5% = 0.025)

is

dB = 6.5 + 9 [log10(F)] + 9 [log10(BW)].

Using a 96- and 480-MHz clock with a 2.5% spread, the theoretical dB reduction would be

dB @ 96 MHz (Fund) = 6.5 + 17.84 - 14.4 = 9.92 and
dB @ 480 MHz (5th) = 6.5 + 24.13 - 14.4 = 16.21.

Regulatory agencies control the maximum amount of radiated radio-frequency (RF) energy. Unwanted RF energy is considered EMI, which causes interference in local receiving equipment such as televisions, radios, cell phones, and pagers.

The Federal Communications Commision has two classes of radiation levels, stated as Class A and Class B. Class A devices are digital devices intended for use in commercial, industrial, or business applications. Class A devices are not intended for use by the general public or in the home. Class B digital devices are intended to be used in the home, but can also be used elsewhere. Class B levels are harder to meet than Class A.

Table I lists the voltage levels allowed under FCC Part 15, for both Class A devices at 10 m and Class B devices at 3 m.

If the equipment under test exceeds these limits, the excess energy must be reduced to within agency limits. Reducing the excess amount of EMI to just under the agency limits is dangerous because there is no guarantee that the differences in manufacturing and environmental changes might cause the energy to increase slightly.

Most companies require a safety margin to ensure that the device always complies with agency limits even when manufacturing processes or environmental conditions change. Reducing a particular offending frequency that is, say, 10 dB over the limit at the 5th harmonic can be very difficult. The problem is further complicated by a company-imposed 4-dB safety margin.

Figure 1. The 5th harmonic of a 96-MHz clock has been reduced by greater than 15.5 dB just by replacing the original clock oscillator with a low-EMI clock oscillator.

Replacing the original clock source with a low-EMI clock oscillator is the most efficient way to systemically reduce EMI by a significant amount. Referring to Figure 1, the 5th harmonic of a 96-MHz clock has been reduced by greater than 15.5 dB just by replacing the original clock oscillator with a low-EMI clock oscillator.

Conclusion

Spread-spectrum technology can be used to reduce EMI. By modulating the carrier frequency, the energy is spread out over a wider range of frequencies, thereby reducing the peak energy contained in any one frequency. Low-EMI clock oscillators are ideal when EMI needs to be reduced by as much as 20 dB. Replacing the original clock source with a low-EMI clock oscillator is the most efficient way to systemically reduce EMI.
 
Tom Tritthart is application engineer for Abracon Corp. (Aliso Viejo, CA). He can be reached at 949-448-7070 or via the company's Web site at http://www.abracon.com.