IAN LANG ELECTRONICS
Repeaters and Restorers.
Earlier in our studies we looked at losses in the cable due to dispersion. The amount of loss in a cable is known as the attenuation factor and is measured in terms of decibels per kilometre. A loss of 3 decibels (dB) is equal to a halving of the power. To combat this we can install every so often a receiver/amplifier/transmitter device along the line. In long distance runs this could be once every 40 km or so.
If we were to do the above with an analogue system we would call these repeaters; and we would find that with each repetition we would introduce noise to the system, until we got to the point where the noise overcame the signal. For this reason analogue systems are not recommended over 500 metres. In digital systems we have no such restriction and we call them restorers for reasons which will become plain. As you know a simple digital system consists of off and on. Consider the diagram below which is a heavily modified one originally obtained from PCB Heaven-
On the top you can see what is plainly a digital signal operating in a pulse width modulation. The same principle can be applied to any digital modulation system.. There are obviously two pulses but some noise has been introduced so that the square wave has become irregular. Our linear amplifier at the receiving end of the restorer has magnified the problem. We can now restore the square pattern using a comparator (hence the term restorer) which, when it sees amplitude above the upper threshold, assumes a pulse. This it
sends to the transmitting end. But there is a problem. Our signal dips below and above the threshold in the same cycle. Thus, in what is supposed to be a narrow pulse there are three narrower still, and in what is supposed to be a wide pulse there are two narrower ones. What we need is a comparator that has a lower threshold too, and when the amplitude of a signal crosses the upper it assumes a pulse until the amplitude crosses the lower again. Furthermore if the amplitude crosses only the lower but not the upper, it assumes no pulse. The device we are looking for is a Schmitt Trigger. In the diagram, you can see the difference. despite the fact there are two large downward spikes, the two pulses are restored to their correct widths and passed to the transmitter from where they will enter refreshed to the next stretch of cable. The Schmitt trigger when used in this way is often referred to as the "shaper". So for a digital system we have the chain receiver/amplifier/shaper/ transmitter.