Do I Need a Terminator for Transmission Lines?

In summary, the technician told the person that they needed to terminate the transmission line with the same impedence as the characteristic impedence of the line itself in order to prevent reflections from the end and 'ringing' in the line. The person questioned whether this was actually necessary, as the 50 Ohm termination that was provided should have been enough. However, the RC time constant for this system can be reduced by terminating the line with a 50 Ohm terminator.
  • #1
gareth
189
0
...no, not the cool one, the one found at the end of transmission lines (sorry).

Well here's my question;

I believe you need to terminate transmission lines with the same impedence as the characteristic impedence of the line itself in order to prevent reflections from the end and 'ringing' in the line.

I'm using an oscilloscope to measure electrical signals, the line is standard RG58 co-ax which has a 50 Ohm characteristic impedence. The imput impedence of the scope is 50 Ohm (or so it says on the outside).

In this case do I need a terminator at all?

A technician recently told me you need to put a 50Ohm terminator on the end of the line (a T connector to the scope input, with a terminator on one end and the input on the other) because the impedence usually marked on the scope isn't correct.

But this confuses me, surley then you're measuring the signal across a 25 Ohm resistor rather than a 50 Ohm because the two resistors are in paralell.

The other thing that bothers me is the RC time constant for this system, if you have miles of cable you're increasing the stray capacitance in the cable (~15pF per foot I think), so you get a larger RC time constant with longer cables, agreed? But how does the termination effect this? Can you eliminate the RC time constant to a minimum using termination techniques?


Any info on termination/terminators would be very welcome, searched around the net but nothing very concise is available.

Thanks
Gareth
 
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  • #2
If your oscilloscope has a 50 Ohm input option, then that will work fine for terminating a 50 Ohm transmission line. Just be sure to turn it on.

The wikipedia.org page on transmission lines is a reasonable intro:

http://en.wikipedia.org/wiki/Transmission_line


.
 
  • #3
that's what I thought, but the lab technician didn't seem to trust the scope. when I put the other terminator on it decreased the signal amplitude by 1/2 as you would expect from putting another 50 ohm resistor in parallel with 50 ohm.
 
  • #4
I have had cases where the 'scope 50 Ohm input selection stopped working, but that's rare. You can just verify it with a DVM, since it's a DC 50 Ohm termination that gets switched in. Show that to your lab technician.
 

1. Do transmission lines always require a terminator?

Not necessarily. The need for a terminator depends on various factors such as the length and type of the transmission line, the impedance matching, and the frequency of the signal being transmitted.

2. What is the purpose of a terminator in a transmission line?

A terminator is used to match the impedance of the transmission line, which helps prevent signal reflections and distortion. It also helps to terminate the signal at the end of the line, ensuring that the full signal is transmitted without any loss.

3. Can I use any type of terminator for a transmission line?

No, the type of terminator used should be compatible with the transmission line and the signal being transmitted. For example, a coaxial cable would require a different type of terminator than a twisted pair cable.

4. How do I know if I need a terminator for my transmission line?

You can use a network analyzer or an oscilloscope to measure the impedance of the transmission line and determine if it needs a terminator. You can also consult the manufacturer's specifications or consult with an expert in the field.

5. What are the consequences of not using a terminator for a transmission line?

Not using a terminator can result in signal reflections, which can cause distortion and loss of signal strength. This can lead to poor performance and reliability of the transmission line and the devices connected to it.

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