Emerge - Spring 2008 - (Page 15) Insights A Checklist Table 1 Type RG-11 RG-59 “Precision” Miniature Miniature O.D. 0.285” 0.242” 0.233” 0.125” 0.097” mA for 50’ 80 35 26 17 9 mA for 20’ 200 90 65 43 22 mA for 10’ 400 180 130 86 43 essentially the same current whether clamped around the power cable of Device A, the video cable, or the power cable of Device B. The current required to cause a hum bar depends on the shield resistance of the cable, which is a function of both its construction and length. Table 1 (above) provides a general guideline. Peak loop current is assumed to be 3 times the rms value listed and will produce a 15 mV peak voltage over the length of the cable. Typically, this results in a barely visible hum bar. Higher currents, of course, will make the bar more visible. hum bar regardless of the selector setting because of coupling in the 20-foot cable. Of course, the loop could be broken by defeating the projector safety ground. DON’T DO IT! Defeating safety grounding is both dangerous and illegal - it also makes YOU legally liable should someone be shocked or killed! A safe way to break the ground loop is to install a “ground isolator” somewhere in the signal path of the ground loop. In this case, either a video isolator or a CATV isolator would do the job. For a more details, especially on tradeoffs involved in various solutions, see the handout from one of my full-length seminars at http://www. jensentransformers.com/an/generic%20 seminar.pdf. Power all equipment in the signal chain from a single location. The ideal is plugging all equipment into one outlet or outlet strip. Powering one piece of grounded equipment from a distant outlet or one on another branch circuit tends to create more serious ground loops. Most of the benefit ascribed to “power conditioning” is simply due to plugging all system equipment into the same outlet strip. Obviously, this is always a good idea! Keep cables as short as possible. Longer cables increase the commonimpedance coupling. Use cables with heavy gauge shields. This is especially important when cables must be long. Again, the only property of cable that has any significant effect on a hum bar is shield resistance. In general, smaller diameter “miniature” cables have higher shield resistance and will cause more hum bar problems than larger, standard coaxial cable — miniature cables generally have much poorer high-frequency performance, too. Maintain good connections. Connectors left undisturbed for long periods can oxidize and develop high contact resistance. A hum bar that changes when the connector is wiggled indicates a poor contact. Use a good commercial contact fluid and/or gold plated connectors to help prevent such problems. DO NOT ADD unnecessary grounds. Additional grounding of equipment tends to increase system ground noise current rather than reduce it. Of course, NEVER disconnect a safety ground or lightning protection ground to solve a problem. Use isolators at problem interfaces. Isolators are a “silver bullet” solution for common-impedance coupling, which is the major weakness of unbalanced interfaces, whether audio, video, or data. A Simple Example There is often substantial ground noise voltage between the CATV ground and the ac power safety ground system (see Figure B). This causes a ground loop current flow in the shields of ALL the signal cables between the set-top box and the projector. The voltage drops in the video cables are, generally, in proportion to their lengths. Therefore, this system would likely exhibit a Figure B E M E RG E / S P R I N G 2 0 0 8 15 http://www.jensentransformers.com/an/generic%20seminar.pdf http://www.jensentransformers.com/an/generic%20seminar.pdf http://www.jensentransformers.com/an/generic%20seminar.pdf
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