CN0275

Complete Broadband Video-over-UTP Driver and Receiver Solution for RGB, YPbPr, and More
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Circuit Function & Benefits

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Originally intended to carry local area network (LAN) traffic, unshielded twisted pair (UTP) cable, such as Category-5e (Cat-5e), has become an economical solution in many other signal transmission applications because of its respectable performance and low cost. Among these applications are systems that transport broadband video signals in which three of the four twisted pairs in the cable carry the red, green, and blue (RGB) computer video signals or the luminance and two color difference (YPbPr), high definition component video signals. The required horizontal and vertical synchronization pulses can be embedded in the video signal blanking intervals, or these pulses can be carried as common-mode difference signals among the three pairs. These systems often include video crosspoint switches and are used to distribute video signals from a small number of sources to many displays, as in digital signage applications, or from a large number of sources to a small number of displays, as in keyboard-video-mouse (KVM) networks.

Signals transported over UTP cable suffer from three major impairments that degrade video quality

  • Nonlinear bandlimiting due to the skin effect, resulting in signal dispersion and loss of high frequency signal content. This impairment results in loss of image sharpness and dark streaking.
  • Low frequency flat loss due to resistive loss that reduces image contrast.
  • Delay skew between the four twisted pairs that stems from the unequal twist rates (lay lengths) which are used to minimize crosstalk between the pairs. Delay skew produces color errors in the received image due to the misalignment in time of the three received signals.

The solution shown in Figure 1 overcomes these impairments by using the AD8122 triple receiver/ equalizer to restore the high frequency content of the video signals while also providing flat gain. The AD8120 triple skew compensating, analog delay line adds delay to the two earliest arriving signals such that the three received signals are properly aligned in time. The AD8147 triple driver provides the required single-ended-to-differential conversion of the source video signals.


Figure 1. Equalized and Delay Compensated UTP Driver and Receiver (Simplified Schematic: All Pins, Connections, and Decoupling Not Shown)


Figure 2. Simplified System Block Diagram for Video-over-UTP System

Common Variations

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The lower cost AD8124 triple equalizer is a viable substitute for the AD8122 in systems that only need to drive up to 200 meters of UTP. The AD8124 is not pin-compatible with the AD8122 and is somewhat different in its control functions.


There are a number of options with regard to drivers besides the AD8147. The AD8146 provides the same functionality as the AD8147 but does not include the dedicated sync-on-commonmode circuitry. The AD8146 is usually used in systems that place the vertical and horizontal synchronization pulses in the blanking intervals of the video signal instead of on the common-mode voltages. The AD8148 is the same as the AD8147 but has a fixed gain of four instead of two, and it can be configured to provide pre-emphasis to drive up to 100 feet of UTP. For systems that require lower power consumption, the AD8133 and AD8134 provide the same functionality as the AD8146 and AD8147, respectively, and consume less power, but they have less bandwidth. Finally, for the lowest cost systems that can run on 5 V, the AD8141 and AD8142 CMOS drivers may be the best choice.


UTP installations vary widely and can cover wide areas, pass through multiple patch bays, and at times have no ground reference. These and other conditions can cause large fluctuations in the received common-mode voltage relative to the local receiver ground reference. Placing a flat gain differential receiver with a wide common-mode range, such as the AD8143, in front of the equalizer can provide up to 21 V of input common-mode range in these demanding situations.


The AD8122 and AD8124 both support coaxial cable as well as UTP cable. The AD8122 can be pin strapped to either mode, and the AD8124 uses a VPOLE control to modify its frequency response to support either cable type.

Circuit Evaluation And Test

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Complete system level, plug-and-play driver and receiver evaluation boards are available from Analog Devices, Inc., that contains all of the necessary circuitry video graphics array (VGA) and RJ-45 connectors. Potentiometers with knobs are provided to control equalization and skew correction. The best test configuration is a simple video source, such as a PC and a good quality display. It is preferable that the PC and display are capable of supporting resolutions up to UXGA at 60 Hz.

Equipment Needed


The following equipment is needed:

  • An UXGA video source (laptop computer)
  • The EVAL-CN0275-TX-EBZ transmitter evaluation board
  • The EVAL-CN0275-RX-EBZ receiver evaluation board
  • ±5 V power supplies (two: one for the TX board and one for the RX board)
  • Cat-5e cable, 100 feet through 1000 feet in 100 foot increments (Stellar Labs U5E-24-CMR-665, MCM Electronics #24-10510)
  • An UXGA video display


Test


The simplified block diagram of the test setup is shown in Figure 11. After connecting the equipment, standard video tests can be used to perform end-to-end testing.



Figure 11. Video-Over-UTP Auto Adjust Test Configuration Functional Block Diagram

Sample Products

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Samples

Product

Description

Available Product
Models to Sample

AD8120 Triple Skew-Compensating Video Delay Line with Analog and Digital Control

AD8120ACPZ-R7

AD8122 Triple Differential Receiver with 300m Adjustable Cable Equalization

AD8122ACPZ

AD8147 Triple Differential Driver for Wideband Video

AD8147ACPZ-R7