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What are 50 Ohm terminals used for?

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thilo:
Hi,

what are these 50 Ohm terminals used for that Dave uses in his latest blog? And maybe you could explain to me what this 50 Ohms internal resistance is that Dave talks about?



Thanks,

Thilo.

alm:
Try searching for transmission line theory in any proper EE textbook or online.

Mechatrommer:

--- Quote from: thilo on August 17, 2011, 10:03:30 pm ---what are these 50 Ohm terminals used for that Dave uses in his latest blog? And maybe you could explain to me what this 50 Ohms internal resistance is that Dave talks about?

--- End quote ---
if you have signal source with its output impedance of 50ohm, you need to connect it to another 50ohm internal input impedance device to get maximum power transfer. my 2cnt i just learnt.

ejeffrey:
A 50 ohm terminator is just a little bnc cap that has a resistor (you guessed it, 50 ohms) between the signal and the shield / ground.

Basically, you need the terminating resistor whenever your cable is a 'transmission line' -- that is, the propagation time of your signal cannot be considered instantaneous.  1 meter of coax has a propagation delay of 5 nanoseconds and a round-trip time of 10 nanoseconds.  If your signal changes fast enough that you cannot always consider the input and output to be at the same voltage you will have a problem due to signal reflections.  The exact cutoff where you need proper termination depends a bit on your application, the driving circuit, and whether there are any intermediate 'taps', but basically even a relatively small fraction of a wavelength can cause problems.

kevyk:
what are these 50 Ohm terminals used for that Dave uses in his latest blog? And maybe you could explain to me what this 50 Ohms internal resistance is that Dave talks about?
[/quote]

These are line terminators that are sometimes used particularly in high frequency applications to terminate the open dangling end of a test cable. This reduces spurious unwanted external (or internal) interference from migrating into your testing set up and messing with your measurements. He was using them just to short the scope inputs together through the built in 50 ohm resistor.

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