Manage EMI from high-speed digital interfaces
The small size and low cost of high-speed serial (HSS) interfaces are particularly valuable for mobile devices that must be small, low power, and lightweight. Electromagnetic interference (EMI) problems arise when the mobile device must communicate with remote networks, because the data rates used in modern HSS interfaces often exceed the wireless communication frequencies used in the mobile radios.
To have successful mobile communication products, it is essential that all components within these products coexist while performing their tasks. This means not only that whatever radio signals that are generated unintentionally must not interfere with any intentional radio signals, but it also means that any intentional radio signals must not interfere with the operation of any other circuits. This is called the principle of mutual transparency. The operation of any circuit is transparent – meaning not interfering – to the operation of any other circuit. It is essential that specification development committees pay particular attention to EMI both from the interface into the radios, and from the radios into the interface, because any interface that is either vulnerable or noisy will not provide functioning products however well it might work on its own. MIPI® Alliance has developed two specifications that pay very close attention to mutual transparency.
Whenever electrons are moved around, electromagnetic science tells us (from Maxwell’s Equations) that radio signals will always be generated. Seven major techniques are available to manage EMI at design time. These techniques are isolation, signal amplitude, skew limits, data rate, signal balance, slew rate control, and waveform shaping. These techniques all have different effects, which are discussed in the following sections.
Physical isolation is probably the most obvious technique. Once we have a radio signal, if we can keep it bottled up, then it will not bother anything. Isolation is never perfect though, and at cellular or wireless LAN frequencies, the practical isolation values vary between 20 to 40 dB. Achieving this level of isolation is usually essential to solving EMI problems. Therefore, careful measurement of isolation provided by IC packages and PCB layouts is extremely important.
Reducing the amplitude of the interface signal does lower EMI, but only very slowly. If the signal amplitude is cut in half, then the EMI drops by only 6 dB. This may be enough to get out of a close problem, but this approach also reduces the receiver margin and may lead to interface errors. It is best left as the last resort for these reasons.
Skew and balance
Skew is the time shift between the two components of a differential signal. Balance is the amplitude matching between the two components of a differential signal. These are largely set by the interface driver circuitry, and they are best analyzed together. As Figure 2 shows, when the signal balance is within 10%, its exact value really does not matter compared to the EMI impact from skew. This means that from an EMI point of view it is far more important to minimize skew than to be concerned with amplitude balance when designing interface driver circuits.
The radio spectrum from a digital signal has distinct properties, and from an EMI perspective the most important one is the spectral null at the data rate and its integer multiples. These spectral nulls are clearly seen in Figure 3.
Figure 3 Changing the interface data rate moves the spectral null. This is a particularly effective technique to reduce EMI for a particular band without any need for filtering.
These nulls exist independent of any signal filtering. It is practical to change the data rate then to move a spectral null close to a radio receiver band to remove EMI into the receiver. This is particularly important for GPS receivers, which must work from extremely tiny signals from multiple satellites. Figure 3 shows this technique used to help protect a GPS receiver, changing the data rate from 1.248 Gbps (Fig 3a) to 1.456 Gbps (Fig 3b).