Measurements with the scope or spectrum analyzer. Impedance considerations

neazoi said:

To put it in another perspective, won't the 50R of the generator and the 50R shunt in the scope, be seen as 25R by the scope?

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KlausST said:

Hi,


And where do the 50 Ohms of the cable go?
(just a trick question )

***
The answer is quite simple:
If you are working with a 50 Ohms system you need:
* a 50 ohms source at one and
* a 50 ohms cable
* a 50 ohms termination at the other end

Otherwise you don´t get clean signals. (because of echoing).

***
So for HF the signal: the 50 ohms source impedance and the 50 Ohms cable act as a voltage divider. You see only half of the initial amplitude (inside the source) on the cable. But the cable is finite in length ... thus if the signal travels to the end of the cable you get an impedance jump and thus an echo.
Thus you need to install a termination, which virtually acts as an infinite long cable to prevent from echoing.

Klaus

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