Using Programmable Spread Spectrum Clock Generators for EMI Reduction
By Shuliang Li, Sr. Staff Design Engineer, and Narayan Purohit, Director of Marketing, Cypress Semiconductor Corp.
Electromagnetic interference (EMI) is energy that adversely affects the performance of electrical/electronic equipment by
creating undesirable responses or complete operational failure.
EMI is caused either by radiated electromagnetic fields or conducted voltages and currents. High clock frequencies and shortedge rates in present high-speed digital systems result in EMI problems.
An important source of both conducted and radiated EMI is electrical equipment coupled to AC electrical lines such as
computers, switching power supplies, and electrical devices which utilize electrical motors, such as refrigerators, air
conditioners, and treadmills.
Once EMI from an electrical device is conducted into an electrical wiring circuit, the wiring may act as an antenna and
"broadcast" the conducted EMI as RFI (radio frequency interference) throughout a structure.
Figure 1. Hershey Kiss Spread Spectrum Clock Frequency Profile in Time Domain
The effects of EMI can range from minor nuisances to catastrophic failures and so it is important to effectively control EMI.
Electromagnetic Compatibility (EMC) is the ability of the systems to operate within their intended environment without
conducting or radiating excessive amounts of electromagnetic energy.
EMI Standards and Associated Cost
EMC standards are designed to ensure that items of electronic equipment do not cause problems to each others operation or,
even worse, give rise to equipment malfunctions.
Regulatory requirements regarding EMI shielding for consumer electronics applications " including televisions, radios, portable
entertainment, electronic games, and Internet appliances " varies from country to country.
Using programmable spread spectrum clock generators for EMI Reduction
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EMI Specifications have been issued by various organizations. In the United States, the FCC has issued Part 15, sub-part J,
for Class A and B devices. Class A and level A describe industrial equipment, while Class B and level B are applicable to
consumer equipment. EMI rules reduce interference between electronic devices and address health and safety concerns.
Figure 2. Spread Spectrum Clock Frequency Profile in Frequency Domain
The typical factors considered in the EMI control plan are:
1) PCB Layout " Segregation of Sensitive components, Power & Ground Planes
2) Circuit current " Emissions increase linearly with current
3) Frequency, including slew rates " Emissions increase Square of the Frequency
4) Bandwidth
5) Circuit loop area " To be Held to Minimum
6) Shielding / Filtering " A combination of proper design, filtering and shielding, and other techniques to achieve the required
levels of emissions in the most economical way
7) Spread Spectrum Clock " appropriate spread amount and modulation frequency
8) The ability to dither the center frequency of clocks used in an application system in order to spread radiated emission
energy over a band of frequencies rather than having all the energy emitted at one frequency.
EMI Control & Reduction Techniques
There are two fundamental methods of EMI control and reduction: Suppression and Absorption. The most common methods
of noise reduction include proper equipment circuit design, shielding, grounding, filtering, isolation, separation and orientation,
circuit impedance level control, cable design, and noise cancellation techniques.
These methods require the use of passive and active components such as filters, chokes, ferrite beads, sheets and foils,
shielding along with various PCB layout rules, and Spread Spectrum Clock Generators (SSCG).
Figure 3. Hershey Kiss Spread Profile Advantage
Using programmable spread spectrum clock generators for EMI Reduction
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Attacking EMI Problems at the Source
A fundamental EMC design principle requires that EMI be attenuated at its source on the PCB. Spread-spectrum techniques
are methods by which energy generated in a particular bandwidth is deliberately spread in the frequency domain, resulting in a
signal with a wider bandwidth. A spread spectrum clock generator (SSCG) performs this spreading for designers.
When selecting a spread spectrum clock to attenuate EMI for a consumer product, developers must ensure:
1) The system still passes EMI regulation tests. Good frequency profile and modulation frequency are most important. A
high-quality Hershey Kiss frequency profile gives the best performance in EMI reduction; a triangle frequency profile requires a
larger spread amount to achieve the same EMI reduction (Figure 1 to Figure 3 above). Higher modulation frequency usually
offers higher EMI attenuation (Figure 4 below).
2) System performance is maintained even with spreading side effects. First, the PLL must run at an optimum state, e.g.
high PFD and VCO frequency, appropriate bandwidth, etc. Second, the frequency spread amount usually should be as small
as possible to keep system timing margin high and cycle-cycle jitter low. For down spread, a lower spread amount makes a
system run less slow by not reducing too much on average frequency.
3) Minimal impact on overall system cost. Spread spectrum clock chip price is traditionally a major price factor in consumer
electronics applications. However, as the complexity of consumer systems has goes up dramatically in recent years,
development cost and risk have to be seriously considered as well.
For example, if even one requirement is not met, no matter if it is EMI or jitter performance, applications are more likely to
require a modification to the system clock. The flexibility an of a programmable EMI approach offers insurance by reducing
potential development cost and risk.
Figure 4. Attenuation Optimization with Modulation Frequency
Using programmable spread spectrum clock generators for EMI Reduction
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Spread Spectrum Clock Generators
Spread Spectrum Clock Generators (SSCG) can be categorized into programmable and non-programmable types, as well as
by whether they have a Hershey Kiss and triangular spread frequency profile shape. The spread spectrum clock requirements
for different consumer systems vary in frequency, center or down spread, spread amount, modulation frequency, Hershey Kiss
or triangle profile type, etc.
As non-programmable spread spectrum clock chips are customized for specific applications "offer several fixed selectable
options like frequency ranges and spread amounts" it can be difficult to meet optimum spread requirements while maximizing
cost/performance.
Most fixed function clock chips on the market offer several fixed selectable input frequency ranges (e.g. 20-40MHz, 40-80MHz
and 80-160MHz) and spread percentages (e.g. 0.5%, 1%, 2% and 3%). Optimization requires two sets of PLL parameters "
one set for EMI reduction performance, another for PLL performance.
Figure 5. Frequency Scaling in a GP SSCG Buffer Chip
Using programmable spread spectrum clock generators for EMI Reduction
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Several undesired side effects arise when actual configuration deviates from these optimized settings. For example, when the
input frequency is not at the center of a selection range, VCO and modulation frequency is linearly scaled (Figure 6 below).
The frequency profile may be distorted if PLL bandwidth is low (which it usually is for controlling cycle-cycle jitter, as shown in
Figure 6 below), which impairs EMI performance.
At the low input frequency boundary, the worst happens: because of the low PFD and VCO frequency, cycle-cycle jitter is
significantly increased, and because of the low modulation frequency and possible frequency profile distortion, EMI reduction
is significantly reduced.
Figure 6. Frequency Scaled and Optimized Profiles Comparison
Using programmable spread spectrum clock generators for EMI Reduction
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When spread amount selection is limited, developers will be forced to select a larger than necessary spread amount. This
typically increases cycle-cycle jitter and reduces the timing budget for the system.
If none of the spread percentages satisfies the system, developers must request the clock manufacturer to make design
changes and manufacture a new chip, which at best takes several weeks even for a simple metal layer change and is often
very expensive.
Alternatively, a programmable spread spectrum clock generator provides general purpose clocks supporting field
programmability combined with on-chip non-volatile memory that enables dynamic reconfiguration of spread parameters,
eliminating slow and expensive chip design changes.
Programmability also allows optimization for the best spread spectrum clock performance at desired specifications. For
example, developers can define exact spread amounts like 2.1% (instead of 3% by selection), or optimize modulation patterns
for the desired frequency setting, etc.
Figure 4 earlier shows how modulation frequency optimization, using a 4-PLL clock chip with 2 spread spectrum PLLs can
easily provide an increase of -3 to -4 dB in EMI reduction. Either of the spreading PLLs has two independent spread pattern
selections.
Most developers prefer Hershey Kiss spread clock for better EMI performance, but many clock companies only offer linear
spread clock. Ideally, an SSCG should offer both Hershey Kiss and linear spread clocks. Figure 3 shows Hershey Kiss spread
increased attenuation by -1.67dB for one test case with the 4-PLL clock chip shown above.
Moreover, important clock parameters, like PLL charge pump current, VCO gain, and output drive strength, should be
programmable. Such flexibility greatly improves system performance, reduces system development time, and allows last
minute changes while reducing risk.
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Cypress Semiconductor
198 Champion Court
San Jose, CA 95134-1709
Phone: 408-943-2600
Fax: 408-943-4730
http://www.cypress.com
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