Measurements with the scope or spectrum analyzer. Impedance considerations

[neazoi]

Hi I have an HF (1-30MHz) signal generator which has 50 ohm step attenuator (Pi networks), which I want to measure to a spectrum analyzer or a scope, through a 50 ohm 1 meter coaxial cable.
My scope has a 50R option which shunts it's input to the ground through a 50R. My analyzer has permanently such a 50R shunt resistor.

My question is this:

When measuring this generator with the scope, should I enable the 50R shunt in the scope or not?

In other words, should the setup be generator 50R PI network -> 50R coaxial -> 50R shunt scope input ?

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?

 

[KlausST]

Hi,

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?

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

 

[neazoi]

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

Perfect, simple and clean. Thanks a lot for the explanation!
Source impedance must be equal to the destination impedance and the cable must be of the same impedance, to get meaningful measurements. I won't worry from now on, thanks.