How to reduce the waveform distortion when using RF cable

Thread Starter

vcxo

Joined Aug 20, 2022
6
I am measuring the 100MHz sine wave by a scope. But I found the waveform is distorted (see attached) when I connect the SMA connector on the board to the scope. And the distortion pattern is linked the cable+connector, like cable #1 always have distortion pattern #1, and cable #2 always pattern#. I know the problem is the RF cable and its connector, but how to fix it ? Thank you !
 

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Papabravo

Joined Feb 24, 2006
19,287
I think you need to terminate the cable with its characteristic impedance to avoid the reflections from the unterminated end. The scope lead is unlikely to provide a proper termination. You also may need an alternate measurement strategy.

How long is the cable?
 

Thread Starter

vcxo

Joined Aug 20, 2022
6
The cable is like 2 meter long. What is the correct measurement strategy ? I just need to make the the 100MHz sine wave output from the board is working.
 

Papabravo

Joined Feb 24, 2006
19,287
The cable is like 2 meter long. What is the correct measurement strategy ? I just need to make the the 100MHz sine wave output from the board is working.
The correct measurement strategy is to properly terminate the cable in its characteristic impedance and use a coupler that will sample a portion of the signal without changing the termination impedance. If you don't know how to do that then hire and pay the price for somebody who does.

What is the wavelength of a 100 MHz signal? Is it anywhere in the vicinity of say 2 meters? In that case you can expect all manner of reflections from the unmatched ends of the cable.
 
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