The data transfer rate used in today's high-speed digital interface exceeds a number of operating frequencies of many mobile communication devices (such as smartphones and tablets). The interface needs to be carefully designed to manage the local electromagnetic radiation generated by the interface to avoid interference from other local RF. This paper discusses several most important technologies for controlling high-speed digital interface EMI, how they help solve EMI problems.
Small size and low-cost high-speed serial (HSS) interfaces must be particularly expensive for mobile devices that must be small, low power consumption and light weight. Electromagnetic interference (EMI) occurs when the mobile device must communicate with the remote network, because the data rate used in the modern HSS interface is often higher than the wireless communication frequency used by the mobile device.
In order to achieve successful mobile communication products, all components in these products must have their own position, peaceful coexistence. This not only means any undesirable radio frequency signals must not interfere with any intentional radio frequency signal, but also means that any intentional RF signal must do not interfere with any other circuitry. This is the so-called mutual transparent principle. The operation of any circuit is transparent - this means that it does not interfere with any other circuitry. It is critical that the standard settles must pay special attention to the EMI from the interface to radio, and from radio frequency to the EMI, because the interface can be "alone", as long as it is susceptible to interference or its own emission, the entire product will not Normal work. The MIPI Alliance has developed two specifications that are very concerned about mutual transparency.
Electromagnetic Science tells us (according to Maxwell Equation): When electron moves, a radio frequency signal will be generated. In design, seven major technical management EMIs can be used, which are: isolation, signal amplitude, offset range, data rate, signal equalization, slew rate control, and waveform shaping. These technologies have different functions, and we will discuss one by one.
Isolate
Physical isolation may be the most apparent technology. For RF signals, if we can "shield", it will not interfere with any other signal. Although the isolation will never be perfect, and in the honeycomb or wireless local area network, the actual isolation fails is between 20 and 40 dB. Isolation of this level is often indispensable to solve EMI problems. Therefore, it is very important to carefully measure the isolation of IC packages and PCB layouts.
Figure 1: A possible isolation cover used to represent the radio frequency package.
Signal amplitude
Reducing the magnitude of the interface signal will definitely lower EMI, but the effect is not large. If the signal amplitude is reduced, EMI only reduces 6dB. This may be sufficient to get rid of a closed problem, but the method also reduces the receiver margin and may result in an interface error. Based on this, it is best to use it as a final means to address EMI issues.
Drift and balance
Drift is a time offset between the two components of the differential signal. Balance is a magnitude match between two components of differential signals. These two parameters are basically determined by the interface driver circuit, and it is best to analyze them. As shown in FIG. 2, when the signal balance is within 10%, the exact value of the signal balance is not that important than the EMI of the drift. This means that from the perspective of EMI, when designing the interface drive circuit, minimizing drift is far more than half of the power ratio.
Figure 2: Group contrast of signal balance and drift. This figure shows that management drift is much more important than getting a very close signal balance. Even in 2% of the UI drift, the signal balance error is high in 10%. The signal balance becomes important only when the drift is 100% zero (a unlikely).
Data transfer rate
The radio frequency spectrum of digital signals has different characteristics. From the perspective of EMI, the most important thing is the spectrum zero value of the data rate and its integer multiple. Figure 3, clearly showing these spectrum zero values.
These zero values are independently present in any signal filtering. By changing the data rate, it is a practical selection of EMIs entering the receiver near a radio frequency receiver band near the spectrum zero value. This is especially important for the GPS receiver that must be identified by the extremely weak signal sent back by multiple satellites. Figure 3 shows such a technique for helping to protect the GPS receiver, and the data rate is changed from 1.248 Gbps (Fig. 3A) to 1.456 Gbps (Fig. 3B).
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Figure 3: Changing the interface data rate will move the spectrum zero value. This is a particularly effective way to reduce the specific frequency band EMI without any filtering.
Slew rate
All necessary information carried by the interface is located at the spearnown. Spectrum side petals carry data waveform transform information, not the data itself. For EMIs generated by side lobes (these sidefoot frequencies above the data rate) energy, it can be suppressed by reducing the slew rate of each waveform transform. This is effective because the total bandwidth of the unexpected RF signal is not controlled by the data waveform, but is determined by the fastest transformation (edge) of the data waveform.
Figure 4A (top) illustrates that this technique does affect the "eye diagram" of the interface signal. Although the width of the eye open is narrowed, the separation between the top of the eye and the bottom is not affected. This is the cost of using this filtering technology.
Please note: The wave rate control only reduces the amplitude of the side. Any impact on the main lobe can be ignored. This is advantageous: Benefits, this means that the wave rate control does not dilute the data content. The disadvantage is: only when the interference frequency is from the main lobe, the technology will be invalid. Based on this reason, if applications with M-PHY MIPI Alliance Digrfsm, people tend to use multiple channels that work in lower data rates, rather than a channel working on higher data rates.
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Figure 4: The embossing rate control on the frequency of the frequency of differential signals: the edge transformation time definition of the eye diagram; the bottom) The corresponding spectrum is shown in the figure shown in the map.
Waveform shaping
The direct method of performing the pressure control rate is to adjust the current source charge and discharge capacitance. This creates a linear transformation as shown in FIGS. 3 and 5A below. Other waveform shapes also indeed affect EMI values, and there is a good result. For example, FIG. 5B shows an effect of an index waveform obtained by a simple RC filter. Here, EMI actually becomes more serious. The reason is that when any transformation begins, all of the index waveforms form a sharp corner, even if the end of any transformation is smooth. But in the transformation end, the invasion has occurred.
Figure 5C shows that when all sharp corners are removed from the interface waveform, the spectral clamp performance is greatly improved. The removal sharp corner is the primary goal of waveform plastic, so it is sometimes referred to as a waveform curvature limit.
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Figure 5: Spectrum variation of the EMI signal having different waveform shapes: a) linear transformation, b) index transformation, and C) filtered the waveform. Index transformation actually suppresses the worst ability of EMI.
Technology combination fist
All EMI management techniques begin to maximize physical isolation. In addition to isolation, depending on the specific issues encountered by the Interface Standardization Committee, different technologies will be used. Two examples of the MIPI standards from the announcement are described below.
The MIPI Alliance's M-PHY specification is a HSS link using a low-line differential signal. Since the data transmission rate is higher than that of many honeycombs and other wireless communication frequencies, the data rate selection, slew rate control, and drift boundaries are used to reduce EMIs that occur internally (including possible single-chip) radio frequency receivers. Figure 6 is an example in which this improvement is embodied.
Figure 6: The M-PHY interface of the MIPI Alliance combines the drift boundary and the embossing rate control technology to try to reduce high frequency EMI. This result is compared to the spectrum in Figure 4b.
The MIPI Alliance's RF front end (RFFE) interface has different problems and is used to manage EMI.Rffe applications that require a large single-ended signal, even when the interface is working next to sensitive radio frequency input. The techniques used here are first adopted with a minimum data transmission rate consistent with the application demand. We then perform curvature control over the interface waveform to ensure that any EMI is limited to the operating frequency below the local RF. Figure 7 is an example of demonstrating its effect.
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