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Reduce Electromagnetic Interference

Sparty's Favorite Recipes 014

Peter Alfke

Kiss Your ASIC Good-bye!

Ingredients:

  • Spartan-II Device
  • Spread-Spectrum Clock
  • Instructions

    Most digital systems are clocked with a crystal-controlled frequency. While this stable and predictable frequency simplifies design, debugging, and timing margin analysis, it generates a high level of EMI at the clock frequency and its third and fifth harmonic. (50% duty-cycle signals have no even harmonics).

    Maybe such a stable frequency is not so desirable. Frequency-modulating the clock spreads the radiated energy over a wide band and drastically reduces the energy of any specific frequency. Hewlett Packard’s “Insight”, vol.2, issue 4, 1997, page 6 through 8 gives an excellent tutorial: “Reducing EMI with Spectrum Spread Clocking”. The article, written a few years ago, mentions a 60 MHz Spectrum-Spread Clock by International Microcircuits (IMI SSCG 504)

    Here are three more modern devices from that company: FS741 is a crystal-controlled wobbling oscillator in an 8-pin SOIC package. It accepts crystals between 4 and 70 MHz. SM532 accepts 14 to 70 MHz crystals and can multiply the frequency by several factors from 0.25 to 4, but with an output-frequency range limited to 14 to 120 MHz. SM532 comes in a 16-pin SOIC package. SM530 offers additional control in a 20-pin package. Prices are between $2 and $4. Download datasheets from www.imicorp.comInternet Link

    Spreading the clock frequency ±2.5%, e.g. between 39 and 41 MHz, instead of a fixed 40 MHz, reduces the amplitude of the fundamental frequency, 40 MHz in this case, by 9.5 dB, and the third and fifth harmonic by 11.2 and 13.0 dB respectively. It would take a lot of expensive shielding to achieve a similar reduction.

    Not all systems can tolerate a frequency-modulated clock, most display and telecom systems cannot, but where applicable, a spread-spectrum clock can offer an inexpensive solution to a difficult problem. This method may be of particular interest in Europe where the authorities enforce very tough regulations regarding electro-magnetic radiation.

    If the design requires a 50% duty cycle, the clock can be converted internally back to 50% through the use of the DLL.

     

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