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.com
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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