Classic Datacom, Inc

KVM (keyboard, video and mouse) via CAT5 extenders can be used to remove computers from harsh environments or sensitive, or to restrict physical access to your computer. Extended Video products such as VGA, DVI, S-Video or HDTV via CAT5 extenders are used to extend the video screens. Each extension consists of two units – local and remote units, which are interconnected by a cable CAT5/5e/6. The low cost and ease of installation of twisted pair cables, such as, cat5, cat5e or CAT6 cable, made them a popular choice over traditional coaxial cable for transmission of KVM and AV signals. However, using a type of twisted-pair cable that is not appropriate for your installation environment can lead to irregular performance of the mouse or keyboard, data loss and degradation of image quality, effectively undermine the purpose your workstation or video extension. One of the causes of the video signal and loss of quality when using a twisted pair infrastructure is an installation environment with extreme electromagnetic interference (EMI or also known as radio frequency interference or RFI.)

Electromagnetic interference

Electromagnetic interference is an unwanted disturbance in an electrical circuit by electromagnetic radiation from an external source. All electrical equipment produces electromagnetic radiation during operation. The current flowing in a circuit generates a magnetic field. The potential difference or voltage required for current flow produces an electric field. Strong electric and magnetic fields can cause an electrical device to affect another. When this is not intentional, is known as electromagnetic interference, commonly abbreviated as EMI. electromagnetic interference can disrupt normal operation of electronic equipment. normal levels of EMI such as those generated by computer, switch or router will not affect NTI KVM and video extenders. However, the extreme levels of EMI on the environment can interfere with signal transmission. This can lead to distortion and loss of video image. The longer the cable, and focus less attention to the installation of cables, the more likely the effect of EMI. Some common sources of EMI are in a large building air conditioners, elevator motors, generators, radio transmitters and welders. Installing the cables near large power transformers or near (less than one foot) fluorescent lamps can also cause problems with EMI.

As EMI affect video signals

EMI affects both audio and video signals. In audio signals, environmental information obscure low-level details. In video signals, interference and signal distortion manifests itself in different ways. Figure 1 shows an image without any interference. Figure 2 shows a waveform of the video signal without interference.

Figure 1: Normal File – No interference

Figure 2: Representation of a video signal without interference

The noise from sources such as air conditioners, elevator motors, generators etc. manifests itself as spots or lines of noise, as shown in Figure 3. This is caused by electric and magnetic fields generated in the engine. The bigger engine, bigger and electric field and magnetic fields. Stop the engine and quickly, can cause interference. Figure 4 shows a waveform of the video signal with the interference of a motor.

Figure three: Interference caused by a motor

Figure 4: Representation of interference from the video signal of an engine

Torn and distorted image as shown in Figure 5 is caused by running the cables near sources of strong magnetic fields, such as large power transformers and fluorescent lights. The wires should be close (less than 1 foot) from the fluorescent lights interference. Figure 6 shows a waveform of the video signal is affected by a strong magnetic field.

Figure 5: The interference from fluorescent lights

Figure 6: Representation of interference from the video signal of a magnetic field

A visible pattern of interference lines, as shown in Figure 7 is the result of interference sources such as radio transmitters. The radio signal must be strong and close to allow it to interfere with the video signal is transmitted over a twisted pair. Figure 8 shows a waveform of the video signal with the interference of radio transmitter.

Figure 7: the interference of a radio station

Figure 8: Performance of a video signal transmitter with radio interference

 

Protect video signals EMI

A simple, convenient to prevent EMI from interfering with the signal transmission is to use a shielded cable to transmit the signal. Shielded cables are offered in a wide range of designs. Any type of shielded cable construction offers distinct advantages that should be considered carefully when choosing the best and most cost efficient option for a particular application. Braid Shield: Shield in a braid twisted pair, as shown in Figure 9, provides effective protection solutions in audio and RF ranges. It also provides excellent structural integrity, while maintaining good flexibility. Braid shields are better than aluminum to minimize low-frequency interference and have lower resistance of the DC.

Figure 9: Braid shielded twisted pair cable

Foil Shields, a shield of twisted pair, shown in Figure 10, provides excellent protection against electromagnetic interference and radio frequency audio. Weigh and cost less than braided shields.

Figure 10: Shielded Foil Twisted Pair

Due to its small size, the aluminum shields are commonly used to protect the individual wire pairs from several pairs to reduce crosstalk. Drain wires are usually used with the blade shield termination facility for connecting the socket. Combination Foil / Braid Shields: A screen twisted pair combination paper / braid, shown in Figure 11, provides the most effective shield over the entire frequency spectrum. The role of the combination / braid combines the benefits of a coverage of 100 percent aluminum with strength and low DC resistance of a braid.

Figure 11: Foil and braid screened cable pair Twist

 

How to help reduce the problems of protection against EMI

Reflection and absorption are the main ways in which significantly reduces EMI shielding strength of signal carrying conductors in a twisted pair shielded. Figure 12 is a representation of how the cable may reflect the high frequency EMI, as a radio transmitter. The shield itself also absorb the energy of a radio transmitter EMI, reducing the amount of EMI that makes the inner loop conductors. When properly installed with shielded connectors with ground equipment, shielding redirects a small amount of electromagnetic energy. These effects of reflection, absorption and redirection of protecting highly effective in reducing the problems of high frequency interference.

Figure 12: Representation of Braid or reflective aluminum with EMI shielding

The best way to protect against EMI from low frequency magnetic fields as a large motor or transformer, is to provide sufficient distance between the cable and the source of the field of interference. Cable shielding has only a limited effect to avoid interference problems of low frequency magnetic fields. Instead of a magnetic field to reduce the reflection or absorption, the cable produces a magnetic field instead of reducing the interference of magnetic field frequency. This is the result of the reduction of the variable magnetic field intensity that reaches the pair of conductors. Figure 13 is a representation of how screening can reduce the strength of this type of interference to reach inner conductors twisted pair.

Figure 13: Cable shield reduces the intensity EMI on cable

 

The statement

If you have a problem of electromagnetic noise, we recommend using shielded cable. When it comes to higher frequency electromagnetic noise only, like a radio transmitter, a length of shielded cable to provide adequate protection. For effective protection against low frequency electromagnetic fields, such as an electric motor, a braided shield or a combination of paper / woven screen is mandatory. If you are unsure about the type of electromagnetic noise, you should use a combination of aluminum / braid shielded cable, as it provides the best protection against a wide variety of electromagnetic disturbances.