The Terminator

gareth

...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

berkeman

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


.

gareth

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 paralell with 50 ohm.

berkeman

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.