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 !

 

What kind of cable and how is it terminated?

 

Just direct connect, one side connect SMC connector on the board, the other side connect to scope.

 

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?

 

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.

 

Get a 50Ω feed through like this. It will provide 50Ω to terminate the cable so the coax is not directly connected to the high impedance of the scope.

 

Thank you so much and I will try it next week when back to lab !

 

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.

 

Get a 50Ω feed through like this. It will provide 50Ω to terminate the cable so the coax is not directly connected to the high impedance of the scope.
View attachment 274337

Are we sure the cable is 50Ω? Even if it is not, this will likely be closer than unterminated for sure.

 

Is this the correct onw I need to buy ?
https://www.pulseresearchlab.com/products/prl-ftr-50?variant=29205910280

 

Is this the correct onw I need to buy ?
https://www.pulseresearchlab.com/products/prl-ftr-50?variant=29205910280

If your cable has a characteristic impedance of 50Ω, then yes. You still have not said what the characteristic impedance of your cable actually is.

 

Actually the SMA JACK has 50 ohms
https://www.amphenolrf.com/132134.html

and the cable has 50 ohms too
https://www.vpc.com/my/patchcorddesigner/createPDF.cfm?partnumber=7-339 116 525-096

 

Actually the SMA JACK has 50 ohms
https://www.amphenolrf.com/132134.html

and the cable has 50 ohms too
https://www.vpc.com/my/patchcorddesigner/createPDF.cfm?partnumber=7-339 116 525-096

That's all you had to say. Hope it works for you.