Process Transmission Lines | Types of transmission lines


#1

Signal transmission:

Signal transmission in a process industry is used to transmit process parameters and control signal from and to the process. There are different types of signal transmissions used in the industries. In most of the industries transmission cables are used to transmit data.

The effects of electricity do not propagate instantaneously throughout a circuit, but rather spread at the speed of light. When a pulse signal is applied to the beginning of a two-conductor cable, the reactive elements of that cable begin to store energy. This translates to a current drawn by the cable from the source of the pulse, as though the cable were acting as a (momentarily) resistive load. If the cable under test were infinitely long, this charging effect would never end, and the cable would indeed behave exactly like a resistor from the perspective of the signal source.

Any cable which allows transmitting a signal through it, to a distance, is known as a transmission line.There are different types of transmission based on the connection of transmission lines and propagation of the signal through lines:

  • Open-ended transmission lines

  • Shorted transmission lines

  • Properly terminated transmission lines

Open-ended transmission lines:

The propagation of a voltage pulse forward and back (reflected) on an open-ended transmission line beginning from the time the DC voltage source is first connected to the left-hand end is shown below:

open ended transmission line

The final result is a transmission line that shows the full voltage source, but not the current. This is exactly what we would expect in an open circuit. However, during the time it took for the pulse to travel the length and distance of the line, it extracted the source current equal to the voltage of the source divided by the characteristic impedance of the cable (Isurge = Vsource / Z0). The two-conductor transmission line acted as a load to the voltage source rather than an open circuit. Only after the pulse travelled down the full length of the line and back did the line finally act as a plain open circuit.

Shorted transmission lines:

The following sequence illustrates the propagation of a forward and backward voltage pulse (reflected) on a short-circuit transmission line starting from the moment the DC voltage source is first connected to the left end:

transmission line

The final result is a transmission line that shows the full current of the source (Imax = Vsource/R wire), but there is no voltage. This is exactly what we would expect in a short circuit. However, during the time it took for the pulse to travel along the line and backward, it extracted the current from the source equal to the voltage of the source divided by the characteristic impedance of the cable (Isurge = Vsource/Z0). For a short period of time, the two-conductor transmission line acted as a moderate load to the voltage source instead of a direct short-circuit. Only after the pulse travelled the full length of the line and returned, the line finally acted as a simple short circuit.

Properly terminated transmission lines:

Proper “termination” of a transmission line consists of connecting a resistance to the end(s) of the line so that the pulse “sees” the exact same amount of impedance at the end as it did while propagating along the line’s length.

Propagation of a voltage pulse on a transmission line with proper “termination” beginning from the time the DC voltage source is first connected to the left-hand end:

rom the perspective of the pulse source, this correctly terminated transmission line “looks” just like a line without infinite length termination. There is no reflected pulse, and the DC voltage source “sees” an invariable load resistance all the time.

Data communication cables for digital instruments behave like transmission lines and must be terminated at both ends to avoid signal reflections. Reflected signals (or “echoes”) can cause errors in the data received in a communications network, so proper termination can be so important.