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Cut EMI in digital systems with spread spectrum clock generators (Part 1)

Posted: 01 Sep 2011     Print Version  Bookmark and Share

Keywords:electromagnetic interference  Spread Spectrum Clock Generators  Hershey Kiss 

Over the past couple of decades, applications start becoming digital. Implementation of digital systems is quite simple as it is entirely about logic. However, complexity increases exponentially with signal speed, specifically clock synchronisation, setup and hold time, jitter, and so on.

These problems affect the functionality of not only the individual sub-system, but also cause electromagnetic interference (EMI) when high frequency devices are operating in close proximity.

EMI is an undesirable system response due to either electromagnetic conduction or�radiation�emitted from an external source. This undesirable response or disturbance may interrupt and degrade the effective performance of any electronic system and might cause a complete system failure. Controlling electromagnetic interference (EMI) in any electronic system, therefore, has become an important design issue for electronic system designers.

Most of the problems faced while designing a digital system are directly or indirectly related to the clock of the system. Being the highest frequency signal, high slew rate and periodic nature (usually 50% duty cycle), clock signals become the largest contributor and primary source of EMI.

Furthermore, increasing speed requirements result in radiation with higher electromagnetic energy. To keep control of this radiation, there are several regulatory agencies across the globe which manage various EMI standards to ensure that any electronic equipment does not cause problems to the functioning of other devices.

These agencies set the limit on the maximum allowed radiation emission, which may vary from one country to another. Note that the maximum allowed radiation does not refer to the averaged emission but rather to peak emission. Any single frequency violating this limit will cause a device to fail compliance testing.

Multiple ways have been devised to address EMI and reduce radiated emissions. These include shielding, filtering, isolation, ferrite beads, slew-rate control, and good PCB layout using added power layer and ground planes. These methods can be used individually or in conjunction with others.

While shielding seems to be a relatively simple approach to reducing EMI, it is a mechanical implementation which is expensive and not at all suitable for portable and handheld equipment. Filtering and low slew rate may be an effective approach at low frequencies but not at the signal transitions rates being implemented today. Precise PCB layout techniques, for their part, tend to be time consuming and unique to a system, meaning that one kind of layout technique used in a system may not transfer exactly to another system.

Spread spectrum clocking is another method which can be used effectively to bring down EMI radiations. This article specifically discusses how spread spectrum clock generators (SSCG) can be used to cut down EMI radiations.

Spread spectrum clock generators
With spread spectrum clocking, the concentrated energy of the narrowband clock signal is spread out over a wider bandwidth, reducing the radiated peak emission. Spread spectrum clocking can be visualized as frequency modulation of the input reference clock with controlled frequency deviation (∆f) and modulation rate where the output modulated clock sweeps its frequency repeatedly with time between two fixed frequency points as shown in (figure 1).

Since the total energy contained in the signal remains constant and is distributed over a range of frequencies, the peak emission at any particular frequency is reduced. As the frequency band is made wider, the peak energy is reduced more. A peak EMI reduction of approximately 2dB to 18dB can be achieved using this technique. Such clock generators which generate Spread Spectrum (SS) clock are called Spread Spectrum Clock Generators.

Frequency modulation

Frequency modulation

Figure 1: Frequency modulation of clock signal and EMI reduction: (upper) modulated clock signal; (lower) output spectrum


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