Foreword
MHL (Mobile High-DefiniTIon Link) is an audio and video standard interface developed by Silicon Image, Inc. to connect portable consumer electronic devices. MHL can complete the transmission of audio / video data using only a pair of signal lines (MHL The data transmission rate is 2.25Gbps). In addition, the MHL technology also has the following functions: (1) MHL transmits high-definition video and digital audio to the MHL receiver (receiving device: HDTV, etc.), and from the receiver to the transmitter (source device: portable consumer electronic device, etc.) Provide power for charging; (2) Because there are fewer signal lines required for transmission * 1, there is no need to prepare a dedicated interface such as HDMI on the device, which effectively reduces the number of interfaces on the source device * 2. With the popularization and promotion of portable devices such as digital TVs and smart phones, consumers' demand for transmitting digital high-definition content to personal mobile devices anytime and anywhere is increasing, and MHL will be the mainstream in the future. On the other hand, due to the increasing number of wireless systems equipped with portable devices in recent years, such as GPS, W-LAN, 3G, etc., the electromagnetic interference with these systems has become a major problem we have to face. In order to eliminate these electromagnetic interferences, each interface needs to be properly muted. This article will explain the best solution for MHL sender squelch processing.
Selection Method of Squelch Filter for Differential Transmission Signal Line
While eliminating the noise generated by the differential transmission signal line through the filter, it is necessary to select a filter with high-efficiency squelch performance without affecting the waveform quality. In the noise suppression processing of the differential transmission signal line, the use of a common mode choke coil will be more effective. The common mode choke coil does not produce any impedance for the differential signal, but it is a large impedance for the common mode signal, which can effectively suppress the excessive common mode interference current.
This time, we will use a portable terminal (hereinafter referred to as EUT) that is actually equipped with MHL. We will jointly witness the efficient performance of the common mode choke coil by observing the radiated noise generated by the MHL signal line and the transmitted waveform quality.
Radiated noise generated by MHL signal line
Connect the EUT and the LCD TV with the MHL adapter cable and start measuring the radiated noise generated by the MHL signal line. The effect diagram of MHL is shown in Figure 1, and the measurement result is shown in Figure 2. Since the receiving devices that currently support MHL have not yet been commercialized, we use an adapter cable to convert the MHL output to HDMI. According to the measurement results, we can clearly see that within the range of 30MHz to 1GHz, high radiated noise at intervals of about 75MHz is generated.
Next, the measurement results of the MHL signal waveform are shown in Figure 3. Figure 3 shows the differential and common mode components of the MHL signal. From Figure 3, we can clearly see that not only the differential mode component (differential waveform) but also the common mode component exist in the MHL signal.
Figure 3: MHL signal waveform measurement results
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