AV Technology - January 2009 - (Page 28)

So, fundamentally, we see that the parameters affecting UTP transmission distance are cable attenuation and collective crosstalk. Receiver system gain attempts to overcome cable attenuation while its equalizer adjusts to counteract the slope of cable attenuation over various frequencies. THE GREAT EQUALIZER Cable equalization is a very important function in all digital transmission systems. The concept is to actively control receiver gain at high frequencies in order to compensate for cable loss. The perfect equalizer system creates a “mirror image” of the cable attenuation curve to enhance the receiver’s gain characteristic. In digital transmis- capability. Even with construction enhancements in Category 6 UTP cabling extending network performance to gigabit bandwidth, the effects of crosstalk eventually overtake any performance gains. Shielding and grounding the pairs significantly reduces near-end and far-end crosstalk. Crosstalk elimination extends receiver gain and equalizer performance to their full limit. STP is mandated for Category 7 and ISO Class F and FA. STP cabling supports 600 MHz and 1,000 MHz signaling, thus extending performance from one gigabit to 10 gigabits over a 100 meter link (see sidebar). How does STP compare on attenuation and crosstalk to UTP? Figure 1 compares attenuation and crosstalk values for Category 5e, 6, and 7 cables. First, consider the large margin of crosstalk between The perfect equalizer system creates a “mirror image” of the cable attenuation curve to enhance the receiver’s gain characteristic. sion, this is important since the receiver attempts to level the signal amplitude so that the data bit edge detector (or slicer) can do its job. Cable equalizers have a finite range of compensation capability and are tailored for specific cable types. The serial digital interface (SDI) equalizer is usually designed for specific types of low-loss coaxial cable. For DVI and HDMI, equalizer designs center on twisted-pair cable, whether it may be a dedicated cable or generic Category-style UTP/STP cable. The compensation gain curve (or transfer function) attempts to fit most applications, although it usually is not perfect due to all the cable variations. Why all the fuss with leveling the received signal? The purpose in all this processing is to present the data signal to some form of detection and recovery circuitry for data reconstruction. One common form of reconstruction is the “data slicer.” Data slicers are basically comparator circuits that “look” for edge transitions in the digital data stream by centering the signal on an average DC level, then triggering a threshold comparator to change state as positive-going and negative-going data transitions come along. When the signal level is not equalized, the average DC value drifts around the data slicer set point and this movement is translated at the output as increased signal jitter. Too much jitter corrupts clock and data recovery. STP TO THE RESCUE For a modest cost increase, STP cable greatly extends transmission the Cat 7 crosstalk curve and both the Cat 5e and 6 crosstalk curves. The difference is on the order of about 30 dB. It is obvious that shielding will help us extend transmission distance substantially. Also note the large difference in frequency range. The Cat 5e curve stops at 100 MHz at the TIA network standard limit for that class of service, while Cat 6 service extends to 250 MHz. Of course, these cables do not stop performing at higher frequencies. It is necessary to extrapolate their performance, or measure it, to determine how they work in the application. We see an improved crosstalk margin for Cat 6. Lower crosstalk curves indicate better performance on the graph of figure 1. Cat 7 service extends to 600 MHz in this data example. Even at 600 MHz, the attenuation to crosstalk margin for Cat 7 is about 20 dB. MORE LESSONS FROM FIGURE 1 The data used to create this graph was normalized to represent a 100foot cable. Why? Extending DVI or HDMI over Cat 5e cable is effective to about 100 feet; perhaps a bit longer with Cat 6 cable. With 22 AWG Cat 7 cable, DVI or HDMI 1.2 may function to the full 100 meters. Since the Cat 5e standard stopped at 100 MHz and the nominal distance is 100 feet for extending DVI/HDMI, this length provides a reasonable point of comparison. Recall that, prior to HDMI version 1.3, the maximum TMDS datasignaling rate for DVI and HDMI is 1.65 Gbps. However, the maxiwww.avtechnologyonline.com 28 | AV TECHNOLOGY | january 2009 http://www.avtechnologyonline.com

Table of Contents Feed for the Digital Edition of AV Technology - January 2009

AV Technology - January 2009 Contents Precedent Corporate: Hunkering Down Government: The Great Unifier Education: Technological Self-Sufficiency Integrating Communications into Your Business Process DVI/HDMI Signals over Twisted-Pair Cable AV Helps Explain the Unexplainable AV Enhancements at the Coliseum Product Forum Tech Horizons Product Spotlight New Products Ad Index AV MO

AV Technology - January 2009

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