Issue link: https://iconnect007.uberflip.com/i/1369942
30 DESIGN007 MAGAZINE I MAY 2021 Cable Shield Terminations Another potential issue is if the I/O cable uses a "pigtail" connection to the connec- tor shell (Figure 9). Ideally, cable shields should be terminated in a 360-degree bond for lowest impedance. Pigtails degrade the cable shield effectiveness by introducing a relatively high impedance. For example, a 1-inch pigtail connection has 12 ohms imped- ance at 100 MHz and gets worse the higher you go in frequency, thus defeating the cable shield. is is especially problematic for HDMI cables, because the HDMI working group [5] failed to specify the method for terminating the cable shield to the connector. Fortunately, they are aware of the issue and will better define a proper termination method in the next revi- sion of the standard. In the meantime, there is no guaranty that a particular cable, when used for formal certification testing, will work well, or not. Trial and error of several brands is rec- ommended. Here are the results in testing eight different brands of HDMI cable (Figure 10). Each was driven with a signal generator and measured in an EMI chamber while sweeping the fre- quency, and is detailed in a report. [6] Figure 9: An example of a poor cable shield termination in an HDMI cable. (Image source: Dana Bergey) Figure 10: The results of testing eight HDMI cables from 30 to 1,000 MHz. As you can see, two of these exhibited 25 dB worse emissions across the band. (Image Source: Dana Bergey)