Common Mode Chokes for High Definition Multimedia Common Mode Noise Suppression

1 Introduction

As high-definition products develop towards higher resolutions, a digital differential interface, the so-called High-Definition Multimedia Interface (HDMI), is adopted, which is characterized by enabling digital devices to transmit high-speed video signals. HDMI has little signal attenuation, strong resistance to external noise, and low noise radiation. The reason is that the signal line adopts a mutual demagnetization design. Despite this, problems with radiated noise still occur from time to time, such as common mode noise generated by its internal integrated circuit (IC) and peripheral circuits, or noise converted from differential signals.

In the case of noise generated in this way, the method of using a common mode coil to suppress noise is very effective, because the common mode coil can suppress common mode noise but has little effect on differential signals. However, it should be noted that due to the different frequencies of the signals, the peripheral circuits and components are different, and the selection and mounting of common mode chokes are also different.

With three signal lines and one clock line, HDMI can transmit video and audio signals at the same time. Figure 1 shows a network diagram of the transition-minimized differential signal (TMDS) used in HDMI. Its maximum data transmission rate is 10.2bps, and the frequency of one group of data signals reaches 1GHz or even higher. Initially, the HDMI interface was able to transmit video resolutions up to 1080p or 8-bit true color. The HDMI 1.3 version can transmit images with a resolution of 1080p or 12-bit deep color.

Figure 1 Working principle of HDMI common mode choke

2 Strategies for Suppressing HDMI Radiated Noise

In the process of studying this method, two HDMI devices were connected with a 3-meter-long cable to test the radiated noise from 1 to 5 GHz. Table 1 lists the specifications and parameters of several different types of common-mode chokes, which are DLP11SN900HL2 and DLP11SA900HL2 of the same size (1.25×1.0mm), and DLW21SN900HQ2 of 2.0×1.2mm.

Table 1 Specifications and parameters of several common mode chokes

Now the common mode choke is tested on the signal line and the clock line for noise and thus the influence. The data transmission is evaluated at the highest signal frequency of 1.11 GHz for 1080p or 12-bit deep color. Figure 2 shows the common mode transfer characteristics, especially Scc21, where the noise is eliminated but the amount of Scc21 is increased proportionally, as well as the differential mode transfer characteristics and Sdd21 proportionally affecting the drop in the signal waveform. The evaluation is performed here using a common mode choke.

Figure 3 shows the measurement results of HDMI radiated noise after using a common mode choke. As can be seen from the figure, this common mode choke can suppress noise at 1GHz or even higher frequencies. In fact, it has a strong suppression effect on high-order harmonics. For all the effects of the application of these common mode chokes, the noise is attenuated by about 10db at 2.22GHz. The figure also shows that due to the use of common mode chokes, the attenuation of noise at 3.33GHz and 4.44GHz is diverse. In addition, it can be seen from the figure that the noise suppression effect of DLP11SN900HL2 is worse than that of other models of common mode chokes. This is because the Scc21 value of DLP11SN900HL2 is relatively low at 3GHz and higher frequencies, as shown in Figure 2. From the results shown in Figures 2 and 3, it can be seen that adding a common mode choke with an attenuation capability of more than 10db at 30GHz and above is necessary.

High Definition Multimedia Interface Signal Quality # e#3 Maintaining High Definition Multimedia Interface Signal Quality

Based on the signal waveform evaluation compatible with HDMI 1.3 version, a video signal generator is used as the signal source, and a 12-bit deep color signal is used as the output signal to evaluate the signal waveform state after adding a common mode choke coil and a common electrostatic protection component to this application. Figure 4 shows the change in waveform transmission when the order of installation of each component is different. In this evaluation, a common mode coil DLP11SN900HL2 with a cutoff frequency of 3.5GHz and a 1.0PF capacitor are used as ESD protection devices.

Figure 5 shows the HDMI signal waveform when a common mode choke with a cutoff frequency of about 1GHz is used. At this time, the signal waveform is severely distorted and can no longer meet the HDMI standard. After studying the waveform and the cutoff frequency of the common mode choke, it can be found that only a common mode choke with a cutoff frequency of 3.5GHz or higher can ensure sufficient peripherals to transmit a 12-bit deep color signal. However, engineers should note that the waveform transmission of the electrostatic shielding device will be affected by the value and installation position of the varistor.

4 Impedance matching of common mode choke

Figure 6 shows the impedance of a common mode choke tested with a TDR measuring instrument. The impedance of the component is equal to the characteristic impedance of the transmission line. Impedance matching is necessary to prevent the transmission signal from radiating. Figure 6 also shows the evaluation of the common mode choke for impedance matching, which meets the HDMI standard requirement of 100+/-15Ω. Due to the use of a capacitor of an ESD device, the impedance sometimes fails to meet the requirements of the standard. In this case, the impedance matching standard can be met by adding a common mode choke to an ESD varistor. Figure 7 shows the impedance of a DLP11SN900HL2 added between the HDMI interface and the ESD varistor with a 1.0PF capacitor. The impedance at this time has met the impedance matching standard.

5 Summary

Based on the evaluation model described above, effective noise suppression measures and methods for HDMI 1.3 version or 12-bit deep color signal were determined and implemented. First, the Scc21 value of the common mode choke should be high to suppress the noise in the GHz band, and the Scc21 should be higher than 10db at 3GHz or above. Second, the cutoff frequency of the common mode choke should be higher than or equal to 3.0GHz to ensure sufficient eye signal space. Finally, in order to ensure the anti-static function, the mounting order should be to install the interface first, then the ESD device, and finally the common mode choke. However, impedance matching will become more difficult, which is a problem that must be paid attention to.