Magnetic Field



Magnetic Field

BY: Don McClatchie 

          The new HD-TVI and HD-CVI video formats deliver excellent High Definition video images usually at a lower cost than the IP digital cameras and since they use the same coaxial cable as the old analog cameras it is a simply task to replace them without the need to run new cable.

          The new TVI and CVI HD cameras are still analog in nature with a much wider bandwidth to facilitate High Definition and the new color information that is carried at a much high frequency than the old analog cameras for better color separation. Since this new camera format is still analog it can suffer from all the signal level, crosstalk and interference issues that plagued the old analog systems.

          One of the most common problems originate from ground loops that are caused by both ends of the video cable being grounded to the electrical building grid that allows horizontal bars to appear on the video monitor. Usually two bars across the video monitor but it can manifest itself with more than two bars. One way to test for this type of ground loop is to disconnect the DVR from the camera cable and connect a battery powered monitor to the camera to see if the interference goes away. Then if it does reconnect the ground from the DVR back onto the outside shield of the BNC connector on your battery powered monitor and see if it returns. This type of ground loop requires a cable shield connection from the camera to the DVR with both units grounded at opposite ends of the cable.

          Another ground loop problem that is not so easy to find comes from the use of DC power supplies particularly the type that employ switching technology or switching power supplies. You will know them because they are extremely lightweight in the hand because they lack the heavy iron core magnetic step down transformer used to isolate the power mains from the low voltage side before rectification and filtering to produce the 12 or 24 Volt DC output. DC power supplies of this type will often carry the neutral from the line voltage side through to the negative low voltage side and that will allow a ground loop to flow on the cable. To test for that type of ground loop use a small battery to power the camera and see of the ground loop goes away. Also multi-camera common power supplies can redistribute a grounded cameras ground loop to some or all of the other cameras on the power supply further complicating the problem.

          The third type of interference will look like a ground loop but its origins are quite different. Electromagnetic induction can cause a ground loop type of effect without any common grounding on the coaxial cable.

          Here is an example of electromagnetic induction interference experienced by one of my customers. The job was to install video cameras aboard a large ship, and the rules required all wires and cables to be inside cable trays or conduit, and as is usually the case, no provision had been made for separate video cable runs.

          My customer had no choice but to run the coax cable inside the same trays with the large AC power conductors for the lateral position thrusters, very high current wires to run large horsepower AC motors. He had worked out a way to insulate the coax cable from the high current power wires for electrical safety and fire purposes but his video images would break up and roll with sparkly noise in the picture every time the thrusters were operating.

          The reason the video signal would break up and roll was the fact that the heavy current in the thruster power wires were generating huge magnetic fields in and around the wires in the cable trays that were by proximity being induced magnetically into his video cables and overwhelming his video signals with noisy AC signals many times larger than the desired video signal. Just like a transformer, his coax cable laying parallel in the cable tray was magnetically coupled to the power wires and transferring some of the energy from the thrusters into his cables magnetically.

          Anyone who has tried to wrap a video cable around a power wire or use an exposed power wire to tie wrap their cable to it may have encountered this problem. However, now what can be done to minimize the magnetic interference and get clear images with the motors running?

          Magnetic induction interference is one of the most difficult things to overcome mainly because most forms of coax RF shielding like foils, drain wires and extra braids won’t help to shield the cable from the interfering magnetic field. The field will penetrate most all materials even Mu-Metal has a hard time blocking it. Distance from the wire generating the field will reduce the problem by the square of the distance and when you are doing direct burial next to a conduit with high current AC the further away you can get your cable from the conduit the better.

          So what is the answer to this customers problem of coax confined next to high current AC wires? In this case the solution was to use twisted pair wire to run his video in the tray. A twisted pair video cable has no shielding at all, however it is twisted for a very good reason, that reason is that any magnetic coupling that manages to reach the wires are cancelled by the twists. Each time the wire wraps over itself it reverses the polarity of the interfering signal and cancels the current being induced in the wire.

          Both wires are being cancelled by virtue of the mechanical twists. The cancellation effect of the twists was enough to solve his problem of magnetic interference. So the unshielded twisted pair wires performed better in this case than the shielded Coaxial cable in this application because of the nature of the interference. Be sure to take the nature of the interference into account when trying to block an interfering signal.

          It is important to note that coaxial cable with full shielding will effectively block out RF signals at high frequencies from hundreds of kilo-Hertz up, even though those signals are also electro-magnetic in nature because of a thing called “skin effect” that causes high frequencies to travel along the surface of a conductor instead of going through it, but low frequencies like line currents (60 Hertz) are less effected by skin effect and penetrate the conductors and shields.

          It is also possible to reduce magnetic interference by inverse cancellation. In that case you make no attempt to block the interfering signal instead you use two cables, one has the video signal on it and the other is a pick up cable terminated at the end to pick up an exact duplicate of the interfering signal with no video on it at all. Both cables must be the same length and connected to a cancellation amplifier where the video signal cable is connected to a positive input of the amplifier and the pick up cable is connected to the negative input of the amplifier. Internal to the amplifier the interfering signal is cancelled from the video signal removing the interfering signal. Inside the amplifier what is common to both the negative and the positive inputs (the interference) is cancelled by the common mode effect. What ever gets into the video cable also get into the pick up cable and by inverting the interfering signal in the amplifier it will be removed completely. This type of amplifier usually has a control to balance adjust the signal for maximum cancellation of the interfering signal.

          My sales department would not be happy if I do not mention that we have products to remove ground loop signals for all of these conditions. Go to our website www.fmsystems-inc.com and look for GB-60, GB0-UTP, GB-1, GB1-UTP, GB464, and GB464-UTP. If you don’t see what you need call me: 800-235-6960, or email me: fmsystemsinc@sbcglobal.net

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