I have written about passive methods on adapting 75 ohm video signals to 100 ohms UTP wiring at Differential video signals on UTP cable section of ePanorama.net and my postings Audio, Video and other signals over UTP, Video over UTP and Video over UTP – commercial balun circuit articles. Possibility to send composite video signals appreciable distances on a low cost twisted-pair wiring saves money: standard 75W RG-59/U coaxial cable costs between 25¢ and 50¢ per foot, but a PVC twisted-pair is only pennies per foot). Active circuits typically allow longer distances than passive circuit. Now it is time to tell how active circuits work.
Color video travels on twisted-pair cable article has this circuit diagram of a quite typical video to UTP adaptation:
The circuit design is quite simple to understand. The system consists of two circuits. The first converts the composite video signal to a differential signal using amplifiers IC1 and IC2. On the receiving end there is one fast operational amplifier IC3 wired to work as a differential amplifier. The circuit has a minimum signal gain of two to compensate for the terminations’ 6-dB signal drop.
This circuit treats the twisted-pair cable as a transmission line that is back-terminated with 75 ohm resistors. The termination method gives 150 ohm output impedance between the wire pair, that is somewhat between the traditional low frequency 600 ohm characteristic impedance and high frequency 100-120 ohm impedance of telephone wire pair.
There are also other circuit examples. Send Color Video 1000 Feet Over Low Cost Twisted-Pair application note shows how video over UTP adapters can be made using LT1190 Op Amp as transmitter and the LT1193 Video Difference Amplifier as receiving end. The circuit example shows a bidirectional implementation where video could be sent to either direction as needed.
The circuit topology for transmitter side is pretty similar in idea to the first circuit, just different amplifiers and somewhat different other component values. This transmitter circuit uses 47 ohms + 47 ohms termination on the transmitting and receiving end, meaning around 100 ohms differential impedance termination for the twisted pair wire.
The receiver side uses special differential input amplifier IC LT1193. This circuit has also option to provide cable compensation at the amplifier’s feedback node. In this case, 1000 feet of twisted-pair is compensated with 1000pF and 50 ohm to boost the –3dB bandwidth of the system from 750kHz to 4MHz.
When distances get longer than few hundred meters, you will really start to need some kind of equalization to keep signal quality good (like it is needed on coaxial cable as well). There are also special ICs designed for this.
Adaptive Equalizer for Video Over Twisted Pair page tells that MAX7474 adaptive equalizer recovers losses incurred in the transmission of a composite video (CVBS) signal over unshielded twisted-pair (UTP) cable. The device fully recovers losses for cable lengths of up to 300m and greatly improves signal quality for cable lengths of up to 600m. The device automatically adapts to all cable lengths for CVBS signals with color burst and allows fixed-equalization settings for video signals without burst.
Equalizer IC receives video over one mile of Category 5 cable article describes a special IC designed for long distance cable compensation. ISL59605 MegaQ video-equalizer IC compensates for high-frequency cable loss as high as 60 dB at 5 MHz and source-amplitude variations. The unit continuously monitors the cable characteristics and tries to compensate them.