FINDING SOURCES OF EXCESS CABLE LOSS IN SDI SYSTEMS BEFORE IT FINDS YOU
FINDING SOURCES OF EXCESS CABLE LOSS IN SDI SYSTEMS BEFORE IT FIND YOU
By: Frank McClatchie
One of the major problems facing users of SDI camera signal transmission systems is that the coaxial cable frequently develops greater loss than the Coaxial Cable Manufacturer claims for their product. This excess loss can creep into a cable in several different ways that have nothing to do with the original theoretical loss that the cable should exhibit.
This can be a problem because of the digital nature of the SDI camera signal. It is the nature of digital signals to work perfectly for various lengths of coaxial cable, until a certain length of cable is reached, and then fail to transmit the camera signal at any greater amount of loss in the cable. Either the picture is “perfect” or it breaks up and wipes out completely with no advanced warning that “the end is near”.
Thus, the major advantage of digitizing a video signal such as the SDI system, is also its greatest disadvantage to the user, in that the user of the system cannot detect excess loss in the transmission system until a slight additional transmission loss results in total failure to transmit the picture. This can be a particularly difficult problem when setting up a remote or temporary cable run at sporting events, etc. What is needed is a simple way to measure coaxial cable loss at the highest frequency that is being used to transmit the camera signal when that cable is first installed to insure, that the cable is will carry the SDI signal without failing. This measurement should include a suitable safety margin of cable loss to insure, that changes in temperature, etc. do not kill the signal during use. So just hooking up the cable and looking at the video image does not guarantee safe operating levels. Excess signal losses can be caused by the following conditions:
Excess loss caused by improperly locked BNC connectors. When BNC connectors are only partially twisted, excess losses can average between a fraction of a dB up to more than one Db even on a connector that “feels solid”, but is not fully locked. Excess loss caused by incorrect installation of the BNC connector. Be sure to follow the connector manufacturers instructions very carefully particularly, with regards to trimming the insulation length and the proper shield wire trimming and compression to insure good grounding of the shield to the connector and solder the center conductor whenever possible, crimping can cause problems.
Excess loss caused by corrosion of the BNC connector. Corrosion can set in to a connector, particularly if the connector is left out in the open, gets rained on, or is slightly loose. Connectors around salt water are prone to this problem.
Excess loss caused by too many BNC connectors in tandem. Each connector will create additional loss on the cable even if properly tightened. Avoid extra connectors in any given cable run as much as possible. Each connector will increase the total cable loss by a significant fraction of a dB.
Excess loss caused by moisture, or even worse, actual rain or sea water entering the connector or a pinhole in the cable jacket. Try to protect any outside cable from the effects of rain or puddles.
Excess loss caused by stretching of the cable, even slightly. Pulling a cable should be avoided wherever possible. It is better to un-reel a cable as you go rather than to set up a cable reel at one end and then pulling the cable to get to the other end. The loss of a cable at 750 MHz will actually be increased by the action of stretching the cable.
Excess loss caused by short radius bends or “kinks” in the cable. Short radius bends in the cable should be avoided as they will increase the loss of the cable. Actual “kinks” in the cable will produce severe excess loss in the cable that is permanent and cannot be cured by straightening the cable out.
Excess loss caused by cable that has been “run over” thus mashing the cable. Steel wheels are the worst for causing this type of damage, but even inflated tires can cause additional loss in a cable that does not go away and is hard to notice.
Excess loss caused by cable that has been overheated at some prior time. High temperatures can cause temporary increases in cable loss, but severe overheating can make the additional loss permanent by deforming the cable insulation material.
Excess loss incurred because the cable is still on a spool. The curved cable has greater loss than when it is laid out straight. Tight curves in the cable can cause additional loss. Even the slight loss of a full turn on a reel can increase when the entire cable is on a reel and there is the inductive effects of the coil as well.
Very few of these conditions will incur failure by themselves on short runs of cable, but when the cable is near maximum length, any of these conditions may be sufficient to cause system failure. Often a cable may be plagued by more than one such condition at the same time, increasing the possibility of system failure.
It is recommended that reels of cable be tested at regular intervals on any cable that is used for temporary camera installations to reveal any damage that may have been done at some prior time, before the same cable is used for another application. Measure the high frequency loss of the cable and keep a cable loss record attached to the reel so that changes in the cable loss may be recorded and dealt with before the next use.
One of the easiest ways this cable loss measurement can be made is to use a true power meter (SDI-2) connected to a high frequency band-pass filter (BPF-600). Together they can accurately indicate excess losses from any of the above conditions using a standard SDI camera for a signal source.
The SDI-2 is a hand-held battery operated true RMS dB reading power meter that when used with a Band-Pass filter can measure the High Frequency signal levels on any cable length. When a record is keep of the cables high frequency capability you will know if a cable is damaged in use and can fix the problem before you send the cable out on another job.
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