Termination

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Overview

The term "termination" is used in digital audio to describe a specific value resistor that is used at the end of a transmission chain to match the input impedance to the output impedance and characteristic impedance of the cable. The purpose of termination is to minimize "reflections" of the incoming signal waveform caused by an impedance miss-match back towards the source. Similar termination at the source serves the same purpose. In some cases it is "fixed" and takes the form of a resistor is present on the circuit board where the signal wires attach from the input connector, or may be selectable via a switch or jumper setting. In other cases, it can take the form of a BNC Terminator for attachment to the input connector using a BNC "T".

Basics

All very high frequency signals travelling on signal conductors are subject to a phenomenon known as “reflections” where part of the energy of the signal is reflected back towards the source by impedance miss-matches at the destination. When the reflected energy reaches the source, part of it is reflected again back towards the destination. The amplitude (size) of the reflection is proportional to the miss-match in impedance and the timing is determined by factors such as cable impedance and length.

The most effective method to minimize this effect is to maintain as constant an impedance as possible throughout the chain by using impedance-matched connectors and cable, and to use terminating resistors at both ends of the chain which are of the same impedance value as the cable and connectors.

The simplest approach is to provide the terminating resistors in every input and output; and this is common in connections like AES3 and S-PDIF digital audio. For example, the AES3 standard for transmission over shielded twisted-pair cable is 110 Ohms, and the standard for S-PDIF transmission over coaxial cable is 75 Ohms.

Another type of signal common in digital audio systems is “Word Clock” which is basically a timing reference at a frequency of one cycle per sample period. Word Clock signals are transmitted over 75 Ohm coaxial cable with BNC connectors.

In the case of BNC Word Clock connections, it is sometime desirable to connect one output to more than one input. If the inputs have fixed termination, this does not work; because the signal level will be reduced too much by multiple terminations. The solution is to either have a selectable termination; which can introduce other impedance issues; or to use external termination in the form of a BNC “terminator.” The advantages of this approach are:

  1. The impedance is more tightly controlled by using only impedance matched connectors.
  2. It is easy to see whether or not the input is terminated.
  3. It is easily re-configured to add or remove devices from the “chain.”
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