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Title.ground.loop.0114



POTENTIAL CAUSES & CURES TO GROUD LOOP INDUCED HUM, (con't);

GROUND LOOP ISOLATOR;

Similar to an audio transformer for unbalanced interfaces, a transformer converts the video signal itself into an ac magnetic field which then induces a replica signal in the secondary winding. Since the two windings are electrically insulated, common-mode impedance is very high at 60 Hz. Likewise, ground voltage is limited only by internal insulation and is commonly 300 volts or more.

DIAGRAM III
SCHEMATIC OF GROUND
LOOP ISOLATOR

DIAGRAM III; SCHEMATIC OF GROUND LOOP ISOLATOR

The biggest advantage of a transformer is that loop current is reduced to negligible levels and hum rejection remains very high even with extreme ground voltage differences and/or very long cables. Transformers are also passive (i.e., power is NOT require of operation) and bi-directional. Their main limitation is bandwidth. Realizable transformers cannot have dc response and are limited to about a 1,000,000:1 frequency range. For NTSC video, poor low-frequency response can cause black level to vary from the top of the image to the bottom this is referred to as field-rate slope. Good performance in this respect requires low-frequency response extending to about 10 Hz. Therefore, practical, state-of-the-art video transformers have good response from about 10 Hz to 10 MHz.

COMMON MODE CHOKE;

Whether called a hum eliminator, hum suppressor or ground loop inhibitor, the most commonly used design for hum bars reduction is the common mode. Although they use windings and core material like a transformer, their construction and operation is fundamentally different.

DIAGRAM IV
SCHEMATIC OF COMMON
CHOKE ISOLATOR

DIAGRAM IV; SCHEMATIC OF COMMON CHOKE ISOLATOR

Referring to the diagram, note that the video signal current flows from appliance I to appliance II through the left side winding and returns through the right side winding. Because the two windings have the same number of turns and the same current flows through them in opposite directions, their magnetic fields cancel and there is no signal-related magnetic field. However, ground loop current flows through the shield and inner conductors in the same direction, nearly all flows in the shield since inner conductor circuit has much higher impedance.


COMMON MODE CHOKE con't;

Therefore, ground loop current
does produce a magnetic field in the core that reacts with the coil to create an inductor (choke), which determines common-mode impedance. The two windings are actually a single length of mini coaxial cable. As previously described, since the choke is now the highest impedance component in the ground loop, most of the ground voltage difference will now appear between its input and output ports. However, there is a limitation on the ground voltage difference. The core material and number of turns used will determine the common-mode voltage at which the core will become magnetically saturated, this will cause the impedance of the choke to be greatly reduced. Therefore, ground voltage differences over a certain level will cause hum rejection to deteriorate or vanish.

As with transformers, common mode design are also passive and bi-directional. The major advantage of a design is the wide bandwidth. Low-frequency response extends to dc and high-frequency response is limited only by the length and type of coaxial cable used in the winding. Typical bandwidths range from 100 to 1,000 MHz, making these units suitable for high-definition TV systems.

ISOLATION AMPLIFIERS

A differential amplifier, such as the simplified one shown in the diagram below has the capability to null its response to common-mode voltage. Of course, at video frequencies, the circuit must be designed carefully to maintain high bandwidth. In typical designs, the common mode impedance is about 1 kS. This relatively low common-mode impedance makes rejection very dependent on the driving source impedances. In a video system, these impedances vary with cable resistance (length) and accuracy of the 75 ohm S source. Therefore, most differential amplifier devices require a trim adjustment to achieve maximum common-mode rejection. This might be a disadvantage in portable systems where cables are frequently changed or re-routed. Although differential amplifiers require power and contain active circuitry, features such as multiple outputs and adjustable gain can be useful.

DIAGRAM V
SCHEMATIC OF ISOLATION
AMPLIFIER

Example of ground loop isolation amplifier

COMMON OBSERVATIONS & CAUTIONS WHEN CORRECTING VIDEO & AUDIO HUM;

PLUG-IN SURGE PROTECTION DEVICES; Plug-in strip surge supressor devices can increase the possibility of electronic product damage. These devices are little more than MOV or metal ozide varistors that shunt surge potentials to the safunding system. When the strip line plugin protector are connected to a different electrical circuits or the interconnecting wiring from other equipment in from a common system. The real possible for very high voltage potentials to be generated in the safety ground system. When this occurs the interface circuitry pf the common system can be damaged when exposed to these high voltage potentials

INSTALL HUM REDUCTION DEVICES AFTER THE SYSTEMS LIGHTNING GROUND & PROTECTORS; oHum reduction devices will be damaged and rendered inoperatable if subjected to high surge voltages and currents.

NEVER REMOVAL A SAFETY GROUND ON 3-PRONG AC PLUG; to effect a possible fix for any hum issues. This creates a very dangerous electrocution possibility when operating the electronic porducts.

CATV SPLITTERS CONNECT SHIELDS OF DIFFERENT DEVICES; and can provide possible paths for ground loop currents. It is suggested that a separate ground loop isolation be affixed to the separate outputs of the splitters.

SATELLITE DISH SYSYRMS MUST BE GROUNDED; this provides protection from lighning. This grounding can cause a ground loop to invade the satellite system just a would for a CATV configured system. BECAUSE GROUND LOOP ISOLATORS DO NOT PASS DC VOLTAGES anyother method must be employed to remove the ground loop. Refer to the manufacturer's installation instructions to create the correct grounding needs in your area.



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6/23/2017