Emerge - Spring 2008 - (Page 14) Insights FINDING GROUND LOOPS THAT CAUSE HUM BARS In video systems, a very common problem is the “hum bar.” Technically, it is a grounding issue, but most designers and installers of video systems think of grounding as a black art. How many times have you heard someone say that a cable is “picking up” noise — presumably from the air like a radio receiver? Or that the solution is “better” shielding? may be even greater between ac power ground and another ground connection, such as a CATV feed. These otherwise insignificant voltages are a problem only when they cause small ac currents to flow in vulnerable video (or audio) signal cables — which is more unfortunate than improper. We must also accept this fact as reality. The shield of coaxial video cable is, of course, the return path for the video signal itself but, as shown in Figure A, it also becomes a path for power-line ground current. The current in this loop depends on the system ground voltage difference and the total resistance in the loop. In accordance with Ohm’s Law, a voltage drop, proportional to the resistance of the shield, will appear over the length of the cable. Because driver (device A) and receiver (device B) impedances are equal, half of this voltage is added to the signal as seen by the receiver. This power line related voltage is what causes a horizontal artifact in the displayed image called a “hum bar.” For standard NTSC video formats, the Bill Whitlock President and Chief Engineer Jensen Transformers, Inc disturbance slowly moves upward. The movement, caused by the 0.06 Hz frequency difference between the NTSC field rate of 59.94 Hz and the 60 Hz power line, takes about 16 seconds to move from bottom to the top of the screen. Because of the nature of the coupling mechanism, a pair of disturbances is most commonly seen. A standard video signal has a total magnitude of 1 V peak-to-peak, of which about 700 mV corresponds to the active black to white display range. An interference voltage of only 7 mV peak-topeak is discernable in most displays under worst-case conditions. Therefore, as little as 14 mV peak-to-peak of voltage difference from end to end on a cable creates a visible problem. AC Power and Ground Loops Most systems consist of at least two devices which operate on utility ac power. Although audio hum and video hum bars are often blamed on “improper grounding,” in most cases there is actually nothing wrong with the system grounding. A properly installed, fully code-compliant ac power distribution system, by its very nature, generates small, entirely safe voltage differences between the safety grounds of all outlets. In general, the lowest voltage differences (a few millivolts) will exist between physically close outlets on the same branch circuit and the highest (up to several volts) will exist between physically distant outlets on different branch circuits. Note that voltages Locating the Coupling Point A quick and easy way to trace ground loop currents, especially handy for multichannel or “component” video, uses a sensitive clamp-on ammeter that can measure low currents with a resolution of 1 mA or better. The one pictured below costs about $250 and is available from Mitchell Instruments as item WB380947 (www.mitchellinstruments.com). This tool allows us to determine the path of, and measure the current in, ground loops by simply clamping the meter around cables — without disturbing the system in any way. In our schematic, we would measure Figure A E ME RG E / S P R I N G 2 0 0 8 14 http://www.mitchellinstruments.com
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