Question

A number of years ago at an ACM SIGCSE Technical Symposium, there was a presentation from a school that created a data communications cart on wheels. This cart, amongst other things, contained a large spool of twisted pair wire, an oscilloscope, and a signal generator. The instructors would use the signal generator to insert a square wave (digital signal) into one end of the spool, and then connect the oscilloscope to the other end of the spool of wire. By observing the received signal on the oscilloscope they would see what happened to the signal as it propagated through the coil of twisted pair wire.

Questions:

1. What happens when we increase the length of wire? Shorten the length of wire?

2. What happens if we increase the data rate of the signal injected into the spool of wire?

Answer 1

Twisted pair cable is a pair of insulated metallic wires twisted around each other. It was discovered and patented by Alexander Graham Bell for the use in telephone transmission.

The signal travelling through a twisted pair cable suffers heavy attenuation, since this is a very old technology for communication

Due to the resistance of the wire, the signal peak intensity will reduce

a) As length increases, resistance increases. So, the peak intensity of the signal will reduce as length increases.As length decreases, the peak intensity will increase and approach the input wave intensity.

b) The data rate depends on the intensity of the input signal.

For an AC signal there is a quantity called skin depth - the distance until which the current flows in a conductor.

In a twisted pair cable,

Where is the frequency.

So, as data rate increases, frequency increases and skin depth decreases.

So, the signal attenuates faster and effectively, the signal strength decreases.