- #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
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