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The Transmission Line Termination Impedance and Signal Reflections

Key Takeaways

  • The magnitude of signal reflections is directly proportional to the severity of the impedance mismatch. 

  • As signal reflections increase, they interfere with input signals and induce noise in a circuit.

  • In RF engineering, the standard impedance is 50 Ω. Co-axial cables and RF components such as ICs, fixed attenuators, and antennas are designed for this standard termination impedance value.

 Data transfer

Transmission line termination impedance is important to obtain quality output, especially when transferring critical information

In RF and microwave circuits such as mixers, amplifiers, and filters, we often discuss the terms input impedance, output impedance, characteristic impedance, matching impedance, and termination impedance. The transmission line termination impedance is particularly significant in RF circuits in order to obtain quality output when transferring critical data. Terminating transmission lines with the appropriate impedance aids impedance matching and prevents signal degradation. If not terminated properly, the reflections of the signals might degrade the signal integrity of the circuit.

The Standard Value of Co-axial Transmission Line Termination Impedance

A mismatch between impedances leads to serious communication errors in RF circuits, as these mismatches cause reflections. The magnitude of these reflections is directly proportional to the severity of the impedance mismatch. As the reflections increase, they interfere with input signals and induce noise in the circuit. 

The fundamental technique of impedance matching is used to establish the source impedance (Zs) equal to load impedance (ZL) and then design the transmission line and other components so that the characteristic impedance, Z0, equals ZL. This method can bring down the signal reflections in the circuit and maximize power transfer.

However, this approach of impedance matching is tedious, and when you modify the circuit slightly, the characteristic impedance changes, calling for the next round of impedance matching procedures. The practical approach to impedance matching is to choose a standard impedance that is universally accepted in RF systems. In RF engineering, the standard impedance is chosen as 50 Ω. Co-axial cables and RF components such as ICs, fixed attenuators, and antennas are designed for this standard termination impedance value. 

The co-axial cables that are abundantly used in RF circuits to apply input signals or to collect the output signal have a characteristic impedance equal to 50 Ω. When we are using 50 Ω to terminate RF circuits, electromagnetic wave reflections are minimized. The use of  50 Ω as the termination impedance also makes the power handling in circuits efficient with low loss. 

Why Use the Value of 50 Ω?

There is nothing special about the value of 50 Ω. Its significance is that it is a globally accepted value among manufacturers and designers of RF circuits and components. In contrast to the universally accepted 50 Ω value, co-axial cables used in television communication systems use 75 Ω as a standard value. 

Signal Reflection in Open Circuited and Short Circuited Transmission Lines

A transmission line can be terminated by an open circuit or short circuit. Let’s see the reflection of signals in the RF circuits terminated with open circuits and short circuits. The reflection coefficient is given by the following equation (1):

Reflection Coefficient

Open Circuit Termination

In open circuit termination, Zt, or the termination impedance is infinity and makes the reflection coefficient equal to +1. The value of +1 indicates total power reflection with an increased voltage at the load end. The voltage is maximum, with minimum current throughout the transmission line. 

Short Circuit Termination

In short circuit termination, Zt is equal to zero. Substituting this value gives a reflection co-efficient equal to -1. The value of -1 indicates that power is reflected and the voltage at the load side is reduced or totally canceled. 

Understanding how transmission line termination impedance works helps designers limit signal reflection and transfer maximum power through impedance matching. Cadence’s software can assist in this impedance matching. 

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